Nullable properties

[cussrox]

This PR fixes #25.

[codecov]

Codecov Report

Merging #26 into master will increase coverage by 0.27%.
The diff coverage is 87.71%.

@@             Coverage Diff              @@
##             master      #26      +/-   ##
============================================
+ Coverage     87.36%   87.63%   +0.27%     
- Complexity      466      493      +27     
============================================
  Files            37       37              
  Lines          1258     1310      +52     
============================================
+ Hits           1099     1148      +49     
- Misses          159      162       +3

Impacted Files	Coverage Δ	Complexity Δ
src/PhpClassProperty.php	62.96% <100%> (+6.44%)	11 <2> (+2)	⬆️
src/PhpClass.php	98.33% <100%> (+0.33%)	21 <0> (+2)	⬆️
src/JsonSchema/PhpBuilder.php	83.43% <83.72%> (-0.54%)	64 <22> (+23)
src/PhpNamedVar.php	63.04% <0%> (+4.34%)	18% <0%> (ø)	⬇️
src/Traits/Description.php	80% <0%> (+20%)	4% <0%> (ø)	⬇️

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