Unmarshalling nullable objects

[p1c2u]

Fixes #232

[codecov]

Codecov Report

Merging #239 into master will decrease coverage by 0.07%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #239      +/-   ##
==========================================
- Coverage   96.86%   96.78%   -0.08%     
==========================================
  Files         122      122              
  Lines        2393     2398       +5     
==========================================
+ Hits         2318     2321       +3     
- Misses         75       77       +2     

Impacted Files	Coverage Δ
...penapi_core/unmarshalling/schemas/unmarshallers.py	94.63% <71.42%> (-1.21%)	⬇️

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