fix: add additional case to _is_scalar_type

rdfproxy/utils/mapper_utils.py

@@ -15,8 +15,16 @@ def _is_list_type(obj: type | None) -> bool:
     return _is_type(obj, list)
 
 
-def _is_scalar_type(t) -> bool:
-    return (not _is_list_type(t)) and (not issubclass(t, BaseModel))
+def _is_scalar_type(obj) -> bool:
+    """Todo: this is clearly buggy. fix for actual implemenation."""
+    if obj is None:
+        return True
+    elif _is_list_type(obj):
+        return False
+    elif args := get_args(obj):
+        return all(_is_scalar_type(o) for o in args)
+    else:
+        return not issubclass(obj, BaseModel)
 
 
 def _is_list_basemodel_type(obj: type | None) -> bool: