Python: fix for Python: parameters mentioned in a SK prompt are not synced wi… (#6807)

fixes #6806

### Motivation and Context

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### Description

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### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] 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
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

python/semantic_kernel/prompt_template/kernel_prompt_template.py

@@ -75,10 +75,8 @@ class KernelPromptTemplate(PromptTemplateBase):
                         # is a named arg block.
                         self._add_if_missing(sub_block.variable.name, seen)
 
-    def _add_if_missing(self, variable_name: str, seen: set | None = None):
+    def _add_if_missing(self, variable_name: str, seen: set):
         # Convert variable_name to lower case to handle case-insensitivity
-        if not seen:
-            seen = set()
         if variable_name and variable_name.lower() not in seen:
             seen.add(variable_name.lower())
             self.prompt_template_config.input_variables.append(InputVariable(name=variable_name))