chore: Rename 'join_nulls' parameter to 'nulls_equal' in join functions

[mcrumiller]

As per Ritchie's request, this brings more consistency to the parameter.

I kept the join_nulls function the same name since in that case it's a verb and it makes sense, but as a parameter name it was a simple rename.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.81%. Comparing base (e801b83) to head (7ebd82d).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/src/frame/join/hash_join/single_keys_dispatch.rs	89.04%	8 Missing ⚠️
crates/polars-stream/src/nodes/joins/equi_join.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21507      +/-   ##
==========================================
- Coverage   79.81%   79.81%   -0.01%     
==========================================
  Files        1592     1592              
  Lines      229266   229299      +33     
  Branches     2623     2623              
==========================================
+ Hits       182991   183014      +23     
- Misses      45670    45680      +10     
  Partials      605      605              

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