Fix NULL handling for in-memory tuple filtering

[antekresic]

During DML decompression, we filter out batches that don't need decompression using in-memory filtering which filters compressed tuples using scankeys. Those scankeys were not detecting NULL values correctly for unique constraints which use NULLS NOT DISTINCT setting. This change uses existing codebase to properly detect those tuples when using SK_ISNULL flag.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.21%. Comparing base (59f50f2) to head (a488025).
Report is 667 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7557      +/-   ##
==========================================
+ Coverage   80.06%   82.21%   +2.14%     
==========================================
  Files         190      231      +41     
  Lines       37181    43380    +6199     
  Branches     9450    10911    +1461     
==========================================
+ Hits        29770    35665    +5895     
- Misses       2997     3392     +395     
+ Partials     4414     4323      -91     

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[akuzm]

This is some creative for loop 😅

[timescale-automation]

Automated backport to 2.17.x not done: cherry-pick failed.

Git status

HEAD detached at origin/2.17.x
You are currently cherry-picking commit 19830632d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .unreleased/pr_7557
	modified:   tsl/src/compression/compression_dml.c
	modified:   tsl/src/compression/compression_dml.h
	modified:   tsl/src/compression/compression_scankey.c
	modified:   tsl/test/shared/expected/compression_nulls_not_distinct.out
	modified:   tsl/test/shared/sql/compression_nulls_not_distinct.sql

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   tsl/src/compression/recompress.c

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