Account for null struct-to-class instances when marshalling from native.

[jkoritzinsky]

Fixes #80

[codecov]

Codecov Report

Merging #81 into master will decrease coverage by 0.09%.
The diff coverage is 30.43%.

@@            Coverage Diff            @@
##           master      #81     +/-   ##
=========================================
- Coverage   58.55%   58.46%   -0.1%     
=========================================
  Files         150      150             
  Lines        9135     9148     +13     
  Branches     1121     1123      +2     
=========================================
- Hits         5349     5348      -1     
- Misses       3396     3406     +10     
- Partials      390      394      +4

Impacted Files	Coverage Δ
...Marshallers/StructWithNativeTypeArrayMarshaller.cs	6.57% <0%> (ø)	⬆️
...ator/Marshallers/StructWithNativeTypeMarshaller.cs	72.47% <100%> (ø)	⬆️
SharpGen/Generator/Marshallers/MarshallerBase.cs	50.17% <23.52%> (-1.69%)	⬇️
SharpGen/Model/CppElement.Extensions.cs	44.23% <0%> (-1.93%)	⬇️
SharpGen/Parser/CppParser.cs	75.15% <0%> (-0.16%)	⬇️
SharpGen/Model/CsMarshalBase.cs	70.83% <0%> (ø)	⬆️

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