When storing any document which contains nulls using the StorageService the server returns the error code 500 and an error that the json string was invalid.
This behaviour should be possible, as stated on Json.org "A value can be a string in double quotes, or a number, or true or false or null, or an object or an array. These structures can be nested."
An example of a json document with nulls would be:
"requiredField":"Some value must be here",
Or in an array:
The problem occurs when trying to insert a document or update/save it when it has any JSONObject.NULL value stored.