Fixing logic in custom or HB helper (#4804)

### Motivation and Context

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The custom `or` helper had a bug where it was checking for null values,
rather than checking for non-null values. This PR changes the condition
to `not null`.

Also, renamed a heading in the CreatePlan prompt to better guide the
model in identifying Handlebars built-in _block_ helpers.

### Description

Additional changes to the `GetArgumentValue` method in
`KernelHelperUtils.cs` to return null if variable isn't defined.

Before, if the template referenced a variable that wasn't defined in
`KernelArguments`, the method would just return the key as a string.
Now, it just returns null (as expected).
> I had it returning a string before because I thought Handlebars would
default the argument to its string representation since it templates
regular text, but I double checked [HB
docs](https://handlebarsjs.com/guide/expressions.html#helpers-with-multiple-parameters),
and generally, string literals are surrounded by quotes when a raw value
is passed as an argument.

Should resolve these issues:
- https://github.com/microsoft/semantic-kernel/issues/4661
- https://github.com/microsoft/semantic-kernel/issues/4732

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Gina Triolo <[email protected]>

dotnet/src/Extensions/PromptTemplates.Handlebars/Helpers/KernelHelperUtils.cs

@@ -41,7 +41,7 @@ internal static class KernelHelpersUtils
         // Thus, we try to get the value from the kernel arguments dictionary.
         if (argument is UndefinedBindingResult result)
         {
-            return kernelArguments.TryGetValue(result.Value, out var variable) ? variable : result.Value;
+            return kernelArguments.TryGetValue(result.Value, out var variable) ? variable : null;
         }
 
         return argument;

dotnet/src/Extensions/PromptTemplates.Handlebars/Helpers/KernelHelpers/KernelSystemHelpers.cs

@@ -136,7 +136,7 @@ internal static class KernelSystemHelpers
                 return arg switch
                 {
                     bool booleanArg => booleanArg,
-                    _ => arg is null
+                    _ => arg is not null
                 };
             });
         });

dotnet/src/IntegrationTests/Planners/Handlebars/HandlebarsPlannerTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.ComponentModel;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Configuration;
 using Microsoft.SemanticKernel;
@@ -30,7 +31,7 @@ public sealed class HandlebarsPlannerTests : IDisposable
 
     [Theory]
     [InlineData(true, "Write a joke and send it in an e-mail to Kai.", "SendEmail", "test")]
-    public async Task CreatePlanFunctionFlowAsync(bool useChatModel, string prompt, string expectedFunction, string expectedPlugin)
+    public async Task CreatePlanFunctionFlowAsync(bool useChatModel, string goal, string expectedFunction, string expectedPlugin)
     {
         // Arrange
         bool useEmbeddings = false;
@@ -39,7 +40,7 @@ public sealed class HandlebarsPlannerTests : IDisposable
         TestHelpers.ImportSamplePlugins(kernel, "FunPlugin");
 
         // Act
-        var plan = await new HandlebarsPlanner().CreatePlanAsync(kernel, prompt);
+        var plan = await new HandlebarsPlanner().CreatePlanAsync(kernel, goal);
 
         // Assert expected function
         Assert.Contains(
@@ -51,14 +52,14 @@ public sealed class HandlebarsPlannerTests : IDisposable
 
     [RetryTheory]
     [InlineData("Write a novel about software development that is 3 chapters long.", "NovelChapter", "WriterPlugin")]
-    public async Task CreatePlanWithDefaultsAsync(string prompt, string expectedFunction, string expectedPlugin)
+    public async Task CreatePlanWithDefaultsAsync(string goal, string expectedFunction, string expectedPlugin)
     {
         // Arrange
         Kernel kernel = this.InitializeKernel();
         TestHelpers.ImportSamplePlugins(kernel, "WriterPlugin", "MiscPlugin");
 
         // Act
-        var plan = await new HandlebarsPlanner().CreatePlanAsync(kernel, prompt);
+        var plan = await new HandlebarsPlanner().CreatePlanAsync(kernel, goal);
 
         // Assert
         Assert.Contains(
@@ -68,6 +69,51 @@ public sealed class HandlebarsPlannerTests : IDisposable
         );
     }
 
+    [Theory]
+    [InlineData(true, "List each property of the default Qux object.", "## Complex types", @"### Qux:
+{
+  ""type"": ""Object"",
+  ""properties"": {
+    ""Bar"": {
+      ""type"": ""String"",
+    },
+    ""Baz"": {
+      ""type"": ""Int32"",
+    },
+  }
+}", "GetDefaultQux", "Foo")]
+    public async Task CreatePlanWithComplexTypesDefinitionsAsync(bool useChatModel, string goal, string expectedSectionHeader, string expectedTypeHeader, string expectedFunction, string expectedPlugin)
+    {
+        // Arrange
+        bool useEmbeddings = false;
+        var kernel = this.InitializeKernel(useEmbeddings, useChatModel);
+        kernel.ImportPluginFromObject(new Foo());
+
+        // Act
+        var plan = await new HandlebarsPlanner().CreatePlanAsync(kernel, goal);
+
+        // Assert expected section header for Complex Types in prompt
+        Assert.Contains(
+            expectedSectionHeader,
+            plan.Prompt,
+            StringComparison.CurrentCulture
+        );
+
+        // Assert expected complex parameter type in prompt
+        Assert.Contains(
+            expectedTypeHeader,
+            plan.Prompt,
+            StringComparison.CurrentCulture
+        );
+
+        // Assert expected function in plan
+        Assert.Contains(
+            $"{expectedPlugin}-{expectedFunction}",
+            plan.ToString(),
+            StringComparison.CurrentCulture
+        );
+    }
+
     private Kernel InitializeKernel(bool useEmbeddings = false, bool useChatModel = true)
     {
         AzureOpenAIConfiguration? azureOpenAIConfiguration = this._configuration.GetSection("AzureOpenAI").Get<AzureOpenAIConfiguration>();
@@ -110,6 +156,24 @@ public sealed class HandlebarsPlannerTests : IDisposable
     private readonly RedirectOutput _testOutputHelper;
     private readonly IConfigurationRoot _configuration;
 
+    private sealed class Foo
+    {
+        public sealed class Qux
+        {
+            public string Bar { get; set; } = string.Empty;
+            public int Baz { get; set; }
+
+            public Qux(string bar, int baz)
+            {
+                this.Bar = bar;
+                this.Baz = baz;
+            }
+        }
+
+        [KernelFunction, Description("Returns default Qux object.")]
+        public Qux GetDefaultQux() => new("bar", 42);
+    }
+
     public void Dispose()
     {
         this._testOutputHelper.Dispose();

dotnet/src/Planners/Planners.Handlebars/Handlebars/CreatePlanPrompt.handlebars

@@ -72,7 +72,7 @@ What's the sum of 5+10+15?
 {{#message role="user"}}
 The following helpers are available to you:
 
-## Out-of-the-box helpers
+## Built-in block helpers
 - `\{{#if}}\{{/if}}`
 - `\{{#unless}}\{{/unless}}`{{#if allowLoops}}
 - `\{{#each}}\{{/each}}`{{/if}}