fix(event_handlers): omit explicit None HTTP header values (#1793)

Author: leandrodamascena

Diff for: aws_lambda_powertools/shared/headers_serializer.py

@@ -41,6 +41,10 @@ def serialize(self, headers: Dict[str, Union[str, List[str]]], cookies: List[Coo
         # Duplicate headers are combined with commas and included in the headers field.
         combined_headers: Dict[str, str] = {}
         for key, values in headers.items():
+            # omit headers with explicit null values
+            if values is None:
+                continue
+
             if isinstance(values, str):
                 combined_headers[key] = values
             else:
@@ -60,13 +64,15 @@ def serialize(self, headers: Dict[str, Union[str, List[str]]], cookies: List[Coo
         https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#multi-value-headers-response
         """
         payload: Dict[str, List[str]] = defaultdict(list)
-
         for key, values in headers.items():
+            # omit headers with explicit null values
+            if values is None:
+                continue
+
             if isinstance(values, str):
                 payload[key].append(values)
             else:
-                for value in values:
-                    payload[key].append(value)
+                payload[key].extend(values)
 
         if cookies:
             payload.setdefault("Set-Cookie", [])
@@ -98,6 +104,10 @@ def serialize(self, headers: Dict[str, Union[str, List[str]]], cookies: List[Coo
             payload["headers"]["Set-Cookie"] = str(cookies[-1])
 
         for key, values in headers.items():
+            # omit headers with explicit null values
+            if values is None:
+                continue
+
             if isinstance(values, str):
                 payload["headers"][key] = values
             else:

Diff for: tests/functional/test_headers_serializer.py

@@ -145,3 +145,21 @@ def test_single_value_headers_with_multiple_header_values_warning():
         payload = serializer.serialize(cookies=[], headers=headers)
 
     assert payload["headers"]["Foo"] == headers["Foo"][-1]
+
+
+def test_http_api_headers_serializer_with_null_values():
+    serializer = HttpApiHeadersSerializer()
+    payload = serializer.serialize(headers={"Foo": None}, cookies=[])
+    assert payload == {"headers": {}, "cookies": []}
+
+
+def test_multi_value_headers_serializer_with_null_values():
+    serializer = MultiValueHeadersSerializer()
+    payload = serializer.serialize(headers={"Foo": None}, cookies=[])
+    assert payload == {"multiValueHeaders": {}}
+
+
+def test_single_value_headers_serializer_with_null_values():
+    serializer = SingleValueHeadersSerializer()
+    payload = serializer.serialize(headers={"Foo": None}, cookies=[])
+    assert payload["headers"] == {}