Deserialiser incorrectly deserialises empty NewImage Map and throws AttributeError

[ghost]

This was an issue recently discovered when trying to set up a DynamoDB integration with Stitch. The stream was incorrectly configured to use ONLY_KEYS, however the error message received by Stitch was

Extraction failed for ‘NoneType’ object has no attribute ‘items'

this itself is a python error message received when the following in log_based.py line 90 is executed:
record_message = deserializer.deserialize_item(record['dynamodb'].get('NewImage'))

which calls

def deserialize_item(self, item):
        return self.deserialize({'M': item})

This makes boto3 TypeDeserializer deserialise a Map value like the following {'M': None} in the following Types.py function

def _deserialize_m(self, value):
        return dict([(k, self.deserialize(v)) for k, v in value.items()])

Thus causing the above error message.

To avoid this there should be type checks within the deserialize_item function and an appropriate call to the correct deserialize ie. {'Null': True} for None - since boto3 appears to require: "Value must be a nonempty dictionary whose key is a valid dynamodb type"

This also explains why the following check is never evaluated in log_base.py line 91:

if record_message is None:
                LOGGER.fatal('Dynamo stream view type must be either "NEW_IMAGE" "NEW_AND_OLD_IMAGES"')
                raise RuntimeError('Dynamo stream view type must be either "NEW_IMAGE" "NEW_AND_OLD_IMAGES"')