fix for "'NoneType' object has no attribute 'setdefault'"

[jaccol]

the fix for #365 gave an error:
'NoneType' object has no attribute 'setdefault'

This small patch should fix that :)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ea6aeff). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #366   +/-   ##
=========================================
  Coverage          ?   54.88%           
=========================================
  Files             ?       32           
  Lines             ?     6011           
  Branches          ?        0           
=========================================
  Hits              ?     3299           
  Misses            ?     2712           
  Partials          ?        0

Impacted Files	Coverage Δ
pymisp/api.py	42.19% <100%> (ø)

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[coveralls]

Coverage remained the same at 54.883% when pulling 00bce80 on jaccol:patch-1 into ea6aeff on MISP:master.

[Rafiot]

Thank you!