additionalProperties without schema should allow null.

[Wim-De-Clercq]

Issue #442

[codecov]

Codecov Report

Merging #444 (e017634) into master (4055a36) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #444      +/-   ##
==========================================
+ Coverage   92.88%   92.98%   +0.09%     
==========================================
  Files         100      103       +3     
  Lines        2447     2480      +33     
  Branches      216      220       +4     
==========================================
+ Hits         2273     2306      +33     
  Misses        129      129              
  Partials       45       45              

Impacted Files	Coverage Δ
...penapi_core/unmarshalling/schemas/unmarshallers.py	89.75% <100.00%> (ø)
openapi_core/exceptions.py	100.00% <0.00%> (ø)
openapi_core/__init__.py	100.00% <0.00%> (ø)
openapi_core/util.py	100.00% <0.00%> (ø)

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