add None response case

Author: joeyzhao2018

tests/test_wrapper.py

@@ -547,7 +547,7 @@ def lambda_handler(event, context):
 
     @patch("traceback.print_exc")
     def test_different_return_type_no_error(self, MockPrintExc):
-        TEST_RESULTS = ["a str to return", 42, {"value": 42}, ["A", 42]]
+        TEST_RESULTS = ["a str to return", 42, {"value": 42}, ["A", 42], None]
         mock_context = get_mock_context()
         for test_result in TEST_RESULTS: