[RQL] GROUP_CONCAT bug if None

let's assume a rset with : [{'name': 'toto', 'description': 'loutre'}, {'name': 'toto', 'description': None}, {'name': 'toto', 'description': 'lapin'}]

If we add a GROUP_CONCAT in the RQL to get the description, the result will be wrong.

RQL("Any X, GROUP_CONCAT(D) GROUPBY X WHERE E name X, E description D") => [{"X": "toto", "D": "loutre"}] There is no "lapin" since there is None before.