Add null check for `Partition`

[viceroypenguin]

This introduces a null-check in order to fail-fast on a null resultSelector value.

[codecov[bot]]

Codecov Report

Merging #896 (e6745b8) into master (b31c7fd) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #896   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         110      110           
  Lines        3441     3442    +1     
  Branches     1020     1021    +1     
=======================================
+ Hits         3179     3180    +1     
  Misses        200      200           
  Partials       62       62           

Impacted Files	Coverage Δ
MoreLinq/Partition.cs	98.27% <100.00%> (+0.03%)	:arrow_up:

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