fix(event_handler): revert regression when validating response (#6234)

Reverting regression

Author: leandrodamascena

aws_lambda_powertools/event_handler/middlewares/openapi_validation.py

@@ -136,9 +136,8 @@ def handler(self, app: EventHandlerInstance, next_middleware: NextMiddleware) ->
             return self._handle_response(route=route, response=response)
 
     def _handle_response(self, *, route: Route, response: Response):
-        # Check if we have a return type defined
-        if route.dependant.return_param:
-            # Validate and serialize the response, including None
+        # Process the response body if it exists
+        if response.body and response.is_json():
             response.body = self._serialize_response(
                 field=route.dependant.return_param,
                 response_content=response.body,

tests/functional/event_handler/_pydantic/test_openapi_validation_middleware.py

@@ -1128,76 +1128,3 @@ def handler(user_id: int = 123):
     # THEN the handler should be invoked and return 200
     result = app(minimal_event, {})
     assert result["statusCode"] == 200
-
-
-def test_validation_error_none_returned_non_optional_type(gw_event):
-    # GIVEN an APIGatewayRestResolver with validation enabled
-    app = APIGatewayRestResolver(enable_validation=True)
-
-    class Model(BaseModel):
-        name: str
-        age: int
-
-    @app.get("/none_not_allowed")
-    def handler_none_not_allowed() -> Model:
-        return None  # type: ignore
-
-    # WHEN returning None for a non-Optional type
-    gw_event["path"] = "/none_not_allowed"
-    result = app(gw_event, {})
-
-    # THEN it should return a validation error
-    assert result["statusCode"] == 422
-    body = json.loads(result["body"])
-    assert "model_attributes_type" in body["detail"][0]["type"]
-
-
-def test_none_returned_for_optional_type(gw_event):
-    # GIVEN an APIGatewayRestResolver with validation enabled
-    app = APIGatewayRestResolver(enable_validation=True)
-
-    class Model(BaseModel):
-        name: str
-        age: int
-
-    @app.get("/none_allowed")
-    def handler_none_allowed() -> Optional[Model]:
-        return None
-
-    # WHEN returning None for an Optional type
-    gw_event["path"] = "/none_allowed"
-    result = app(gw_event, {})
-
-    # THEN it should succeed
-    assert result["statusCode"] == 200
-    assert result["body"] == "null"
-
-
-@pytest.mark.parametrize(
-    "path, body",
-    [
-        ("/empty_dict", {}),
-        ("/empty_list", []),
-        ("/none", "null"),
-        ("/empty_string", ""),
-    ],
-    ids=["empty_dict", "empty_list", "none", "empty_string"],
-)
-def test_none_returned_for_falsy_return(gw_event, path, body):
-    # GIVEN an APIGatewayRestResolver with validation enabled
-    app = APIGatewayRestResolver(enable_validation=True)
-
-    class Model(BaseModel):
-        name: str
-        age: int
-
-    @app.get(path)
-    def handler_none_allowed() -> Model:
-        return body
-
-    # WHEN returning None for an Optional type
-    gw_event["path"] = path
-    result = app(gw_event, {})
-
-    # THEN it should succeed
-    assert result["statusCode"] == 422