Add NullArgumentsTest to SuperLinq.Async

[viceroypenguin]

This PR updates SuperLinq.Async to automate null checking on all operators, using a similar test to NullArgumentsTest for the sync version. Updates are also done on operators as necessary to correct shortcomings in null checking.

Fixes #194

[codecov]

Codecov Report

Patch coverage: 97.83% and project coverage change: +1.12 🎉

Comparison is base (b8350cd) 91.82% compared to head (b35d12d) 92.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #476      +/-   ##
==========================================
+ Coverage   91.82%   92.94%   +1.12%     
==========================================
  Files         245      245              
  Lines        7954     8069     +115     
  Branches     1611     1611              
==========================================
+ Hits         7304     7500     +196     
+ Misses        436      359      -77     
+ Partials      214      210       -4     

Impacted Files	Coverage Δ
Source/SuperLinq.Async/CountMethods.cs	96.42% <80.00%> (+0.13%)	⬆️
Source/SuperLinq.Async/CollectionEqual.cs	91.66% <88.88%> (+0.75%)	⬆️
Source/SuperLinq.Async/TrySingle.cs	91.66% <88.88%> (+0.75%)	⬆️
Source/SuperLinq.Async/AggregateRight.cs	92.68% <96.42%> (+7.49%)	⬆️
Source/SuperLinq.Async/FindIndex.cs	94.87% <96.77%> (+0.13%)	⬆️
Source/SuperLinq.Async/GetShortestPath.cs	98.32% <96.80%> (+15.42%)	⬆️
Source/SuperLinq.Async/Case.cs	89.47% <100.00%> (+4.85%)	⬆️
Source/SuperLinq.Async/Do.cs	88.88% <100.00%> (+3.88%)	⬆️
Source/SuperLinq.Async/ElementAt.cs	100.00% <100.00%> (ø)
Source/SuperLinq.Async/EndsWith.cs	100.00% <100.00%> (+9.09%)	⬆️
... and 12 more

... and 8 files with indirect coverage changes

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