[TASK] Add a test to cover undefined fluid variable as argument

This change adds a basic test to show that null-coalsing operator
guard along with a string cast is needed when accessing viewhelper
arguments. Even if registered as required and of type string.

Note: Expected to have a failing functional test now !

The issue should be fixed in a dedicated follow up.

Tests/Functional/ViewHelpers/ExtensionActiveViewHelperTest.php

@@ -53,6 +53,12 @@ public static function renderDataProvider(): \Generator
             [],
             'elseArgument',
         ];
+
+        yield 'extension provided as undefined fluid variable placeholder, await else' => [
+            '<deepl:be.extensionActive extension="{someUndefinedVariable}" then="thenArgument" else="elseArgument" />',
+            [],
+            'elseArgument',
+        ];
     }
     /**
      * @test