Fix optional Model field

Author: electrocucaracha

src/mcpadapt/utils/modeling.py

@@ -145,7 +145,7 @@ def get_field_type(field_name: str, field_schema: Dict[str, Any], required: set)
             default = field_schema.get("default")
             is_required = field_name in required and default is None
 
-            if is_nullable and not is_required:
+            if is_nullable or not is_required:
                 field_type = Optional[field_type]  # type: ignore
 
             return field_type, ... if is_required else default