Add support for None values in list parsing (boto#3618)

* Fix null value handling in list parsing for all protocols

When services return lists containing null values (e.g., RDS Data API
returning arrays with nulls), the parser would fail with TypeError when
attempting to parse primitive types like integers.

The fix adds explicit null handling in _handle_list() to preserve null
values in the parsed output.

Fixes boto/boto3#4508

Author: adev-code

botocore/parsers.py

@@ -352,7 +352,12 @@ def _handle_list(self, shape, node):
         parsed = []
         member_shape = shape.member
         for item in node:
-            parsed.append(self._parse_shape(member_shape, item))
+            # Treat all lists as sparse during parsing to safely handle null
+            # elements that may be present in service responses.
+            if item is None:
+                parsed.append(None)
+            else:
+                parsed.append(self._parse_shape(member_shape, item))
         return parsed
 
     def _default_handle(self, shape, value):

tests/unit/test_parsers.py

@@ -1646,3 +1646,56 @@ def test_can_handle_generic_error_message(parser, body):
     )
     assert parsed['Error'] == {'Code': '503', 'Message': 'Service Unavailable'}
     assert parsed['ResponseMetadata']['HTTPStatusCode'] == 503
+
+
+class TestNullInListParsing(unittest.TestCase):
+    def test_handles_null_in_list(self):
+        parser = parsers.JSONParser()
+        body = b'{"items":[1,null,3]}'
+        output_shape = model.StructureShape(
+            'OutputShape',
+            {'type': 'structure', 'members': {'items': {'shape': 'ItemList'}}},
+            model.ShapeResolver(
+                {
+                    'ItemList': {
+                        'type': 'list',
+                        'member': {'shape': 'ItemType'},
+                    },
+                    'ItemType': {'type': 'integer'},
+                }
+            ),
+        )
+        parsed = parser.parse(
+            {'body': body, 'headers': {}, 'status_code': 200}, output_shape
+        )
+        self.assertEqual(parsed['items'], [1, None, 3])
+
+    def test_handles_null_in_nested_list(self):
+        parser = parsers.RestJSONParser()
+        body = b'{"data":[[{"values":[1,null,3]}]]}'
+        output_shape = model.StructureShape(
+            'OutputShape',
+            {'type': 'structure', 'members': {'data': {'shape': 'DataList'}}},
+            model.ShapeResolver(
+                {
+                    'DataList': {
+                        'type': 'list',
+                        'member': {'shape': 'ItemList'},
+                    },
+                    'ItemList': {'type': 'list', 'member': {'shape': 'Item'}},
+                    'Item': {
+                        'type': 'structure',
+                        'members': {'values': {'shape': 'ValueList'}},
+                    },
+                    'ValueList': {
+                        'type': 'list',
+                        'member': {'shape': 'ValueType'},
+                    },
+                    'ValueType': {'type': 'integer'},
+                }
+            ),
+        )
+        parsed = parser.parse(
+            {'body': body, 'headers': {}, 'status_code': 200}, output_shape
+        )
+        self.assertEqual(parsed['data'][0][0]['values'], [1, None, 3])