fix: Handle null-aware joins correctly in FilterNullJoinKeys when its enabled

[AdamGS]

Which issue does this PR close?

• Closes Optimizer rule FilterNullJoinKeys causes null-aware joins to return incorrect results #23847 .

Rationale for this change

Fixes a correctness bug, not sure when that config used/desirable.

This is another thing I ran into during #21585

What changes are included in this PR?

1. New SLT test
2. Fix for bug in datafusion-optimizer

Are these changes tested?

Existing tests, and a new SLT test verifying the config change doesn't affect the query result and basic unit test.

Are there any user-facing changes?

None

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.48649% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.74%. Comparing base (7fb2e2c) to head (6e30c04).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/optimizer/src/filter_null_join_keys.rs	86.48%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23848      +/-   ##
==========================================
- Coverage   80.75%   80.74%   -0.01%     
==========================================
  Files        1089     1089              
  Lines      368824   368861      +37     
  Branches   368824   368861      +37     
==========================================
+ Hits       297829   297854      +25     
- Misses      53246    53252       +6     
- Partials    17749    17755       +6     

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