ArrayUnmarshaller: return None on nullable type instead of "NoneType object is not iterable" crash

[jgod]

See: #251

Addresses this point specifically:

When nullable is set for arrays (i.e. type: array & nullable: true), null value not being properly validated, instead, it raising error: 'NoneType' object is not iterable. So, the same issue that diogobaeder reported in the second post.

[codecov]

Codecov Report

Merging #276 (22a2771) into master (0aeb144) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   96.80%   96.80%           
=======================================
  Files         122      122           
  Lines        2407     2412    +5     
=======================================
+ Hits         2330     2335    +5     
  Misses         77       77           

Impacted Files	Coverage Δ
...penapi_core/unmarshalling/schemas/unmarshallers.py	94.73% <100.00%> (+0.07%)	⬆️
openapi_core/templating/paths/finders.py	100.00% <0.00%> (ø)

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[p1c2u]

@jgod thanks for the contribution. Looks good. We just need to add tests to cover use cases.