Use unique names for null/non-null groupby columns

[charlesbluca]

This should unblock #286 by ensuring that all groupby columns computed by get_groupby_with_nulls_cols have unique names.

cc @ayushdg

[codecov-commenter]

Codecov Report

Merging #289 (b755bb2) into main (e48d9c1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #289   +/-   ##
=======================================
  Coverage   95.99%   95.99%           
=======================================
  Files          64       64           
  Lines        2797     2798    +1     
  Branches      421      421           
=======================================
+ Hits         2685     2686    +1     
  Misses         71       71           
  Partials       41       41           

Impacted Files	Coverage Δ
dask_sql/physical/utils/groupby.py	100.00% <100.00%> (ø)

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[ayushdg]

Thanks @charlesbluca for the fix. Can confirm that it unblocks my usecase and the changes lgtm!