fix: distinct default namespace values

In some functions, the default namespace was null, while in others was an empty
string. The default values must be consistent for the package to work when a
specific value is not provided.

src/Traits/Taggable.php

@@ -74,7 +74,7 @@ protected static function getTaggedClass(): string
      */
     public function getTagNamespace(): string|null
     {
-        return '';
+        return null;
     }
 
     /**