Handle null-valued ins and outs for miotaction

[rytilahti]

This PR parses null-valued ins & outs for miotactions as empty lists to avoid crashing on unexpected schemas.

Example seen in the wild (roidmi.vacuum.v66):

{
    "iid": 3,
    "type": "urn:miot-spec-v2:action:start-room-sweep:00002826:roidmi-v66:1",
    "description": "Start Room Sweep",
    "in": [
        9
    ],
    "out": null
}

Fixes #1942

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.63%. Comparing base (d517905) to head (cdd4719).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1943   +/-   ##
=======================================
  Coverage   81.62%   81.63%           
=======================================
  Files         197      197           
  Lines       19127    19134    +7     
  Branches     4206     4209    +3     
=======================================
+ Hits        15613    15620    +7     
  Misses       3216     3216           
  Partials      298      298           

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