Add 'np(...)' Null Propagating function

[StefH]

#98

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c8bb7dd). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #223   +/-   ##
=========================================
  Coverage          ?   96.57%           
=========================================
  Files             ?       41           
  Lines             ?     6919           
  Branches          ?        0           
=========================================
  Hits              ?     6682           
  Misses            ?      237           
  Partials          ?        0

Impacted Files	Coverage Δ
.../System.Linq.Dynamic.Core/Parser/KeywordsHelper.cs	100% <100%> (ø)
...ystem.Linq.Dynamic.Core/Parser/ExpressionParser.cs	98.6% <100%> (ø)
...ystem.Linq.Dynamic.Core/Parser/ExpressionHelper.cs	100% <100%> (ø)

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[StefH]

linked to #98