Fix correctly type cast of nulls (and other constants)

[rockResolve]

Add tests that check returned type
Also fixes casts e.g. "int?(5)" to return nullable int type

[codecov]

Codecov Report

Merging #354 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #354   +/-   ##
=======================================
  Coverage   88.49%   88.49%           
=======================================
  Files          46       46           
  Lines        4503     4504    +1     
=======================================
+ Hits         3985     3986    +1     
  Misses        518      518           

Impacted Files	Coverage Δ
...rc/System.Linq.Dynamic.Core/DynamicClassFactory.cs	96.81% <0.00%> (-1.82%)	⬇️
...ystem.Linq.Dynamic.Core/Parser/ExpressionParser.cs	88.15% <0.00%> (+0.31%)	⬆️

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[StefH]

See my comment