Add Assumes.Null and NotNull methods that accept nullable structs

[AArnott]

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[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.41%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   74.07%   74.49%   +0.41%     
==========================================
  Files          10       10              
  Lines         243      247       +4     
  Branches       64       64              
==========================================
+ Hits          180      184       +4     
  Misses         49       49              
  Partials       14       14

Impacted Files	Coverage Δ
src/Microsoft.VisualStudio.Validation/Assumes.cs	73.33% <100%> (+1.9%)	⬆️

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