parse-server: When a cloud function has no result the response is malformed

Issue Description

Cloud functions return a json response formatted as {result: ... } which is then interpreted by the client SDKs But when it should return a undefined result, instead of returning {result: undefined}, it returns {} which is really not a valid json object, and which causes an error in the JS SDK for example.

To ensure the JSON response can be stringify we could use a replacer method to replace all the undefined by null values…? Or we need to ensure all SDKs can cope with this kind of invalid json result…