[BUGFIX] Fix deprecation warning in ContentBlockBuilder (#329)

As of PHP 8.4 implicitly declaring function parameters nullable is deprecated.

Explicitly nullable types have been available since PHP 7.1

See: https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated

Classes/Builder/ContentBlockBuilder.php

@@ -47,7 +47,7 @@ public function __construct(
     /**
      * Writes a Content Block to file system.
      */
-    public function create(LoadedContentBlock $contentBlock, string $skeletonPath = null): void
+    public function create(LoadedContentBlock $contentBlock, ?string $skeletonPath = null): void
     {
         $name = $contentBlock->getPackage();
         $extPath = $contentBlock->getExtPath();