Implement field option to set as nullable for create input type

[erikgaal]

Fixes #78

[codecov[bot]]

Codecov Report

Merging #86 into master will increase coverage by 0.04%. The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #86      +/-   ##
============================================
+ Coverage     84.39%   84.43%   +0.04%     
- Complexity      647      649       +2     
============================================
  Files            75       75              
  Lines          1878     1883       +5     
============================================
+ Hits           1585     1590       +5     
  Misses          293      293

Impacted Files	Coverage Δ	Complexity Δ
src/Fields/Field.php	90.78% <100%> (+0.33%)	66 <2> (+2)	:arrow_up:
src/Types/CreateInputType.php	100% <100%> (ø)	4 <0> (ø)	:arrow_down:

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