Update DensePartialSortBy to use NullKeyDictionary

[viceroypenguin]

This PR updates the DensePartialSort(By) operators to properly use NullKeyDictionary<>; the previous version was copied from PartialSort(By), and the modification did not remove the unnecessary int Index tuple key.

Fixes #499

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (2c09912) 92.99% compared to head (b20cd66) 93.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
+ Coverage   92.99%   93.04%   +0.05%     
==========================================
  Files         249      249              
  Lines        8182     8184       +2     
  Branches     1635     1635              
==========================================
+ Hits         7609     7615       +6     
+ Misses        359      356       -3     
+ Partials      214      213       -1     

Files Changed	Coverage Δ
Source/SuperLinq.Async/DensePartialSort.cs	94.28% <100.00%> (ø)
Source/SuperLinq/Collections/NullKeyDictionary.cs	100.00% <100.00%> (+16.66%)	⬆️
Source/SuperLinq/DensePartialSort.cs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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