fix: [REST] Python has no 'Null' type, it is called 'None'

[SteveClement]

Wrong null type returned by generator.

[coveralls]

Coverage remained the same at ?% when pulling a9a6260 on SteveClement:REST_Client_python into cfa4789 on MISP:2.4.

[Rafiot]

1. We can probably keep PyMISP in the template, as it will be the default in ~90 days when I purge all remaining python 2.7 things
2. Null vs. None, yes, definitely.

[iglocska]

Thanks a lot.

[iglocska]

We can already start preparing users for the switch by letting them generate expanded pymisp scripts by default. If they don't like it, they can ofc manually edit it to pymisp - there's nothing saying these scripts will be running on the same box as MISP itself so we can default to Python 3.6+