Config: Remove default app key value

To avoid scenarios where an instance specific app key is not set, and
therefore avoids the potential of instances using a known key value.

For #6210

app/Config/app.php

@@ -120,7 +120,7 @@ return [
     'auto_detect_locale' => env('APP_AUTO_LANG_PUBLIC', true),
 
     // Encryption key
-    'key' => env('APP_KEY', 'AbAZchsay4uBTU33RubBzLKw203yqSqr'),
+    'key' => env('APP_KEY'),
 
     // Encryption cipher
     'cipher' => 'AES-256-CBC',

phpunit.xml

@@ -17,6 +17,7 @@
     <server name="APP_AUTO_LANG_PUBLIC" value="true"/>
     <server name="APP_URL" value="http://bookstack.dev"/>
     <server name="APP_TIMEZONE" value="UTC"/>
+    <server name="APP_KEY" value="base64:rt54sY5s5G9+8VmvfI0NIJs2Fzd3qsx4auLjnLE5qw0="/>
     <server name="APP_DISPLAY_TIMEZONE" value="UTC"/>
     <server name="ALLOWED_STYLE_SOURCES" value=""/>
     <server name="ALLOWED_IMAGE_SOURCES" value=""/>

tests/Unit/ConfigTest.php

@@ -2,6 +2,7 @@
 
 namespace Tests\Unit;
 
+use Illuminate\Encryption\MissingAppKeyException;
 use Illuminate\Support\Facades\Log;
 use Illuminate\Support\Facades\Mail;
 use Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport;
@@ -212,8 +213,16 @@ class ConfigTest extends TestCase
         });
     }
 
+    public function test_app_errors_if_no_app_key_set()
+    {
+        $this->runWithEnv(['APP_KEY' => null], function () {
+            $this->expectException(MissingAppKeyException::class);
+            $this->get('/');
+        });
+    }
+
     /**
-     * Set an environment variable of the given name and value
+     * Set an environment variable of the given name and value,
      * then check the given config key to see if it matches the given result.
      * Providing a null $envVal clears the variable.
      */