[Bug Fix] SEGV on query with Left Outer Join (#7787) (#7901)

[colm-mchugh]

DESCRIPTION: Fixes a crash in left outer joins that can happen when there is an an aggregate on a column from the inner side of the join.

Fix the SEGV seen in #7787 and #7899; it occurs because a column in the targetlist of a worker subquery can contain a non-empty varnullingrels field if the column is from the inner side of a left outer join. The issue can also occur with the columns in the HAVING clause, and this is also tested in the fix. The issue was triggered by the introduction of the varnullingrels to Vars in Postgres 16 (2489d76c)

There is a related issue, #7705, where a non-empty varnullingrels was incorrectly copied into the query tree for the combine query. Here, a non-empty varnullingrels field of a var is incorrectly copied into the query tree for a worker subquery.

The regress file from #7705 is used (and renamed) to also test this (#7787). An alternative test output file is required for Postgres 15 because of an optimization to DISTINCT in Postgres 16 (1349d2790bf).

[colm-mchugh]

Backport #7901 to 12.1.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.65%. Comparing base (cb31a64) to head (76c8f3e).
Report is 9 commits behind head on release-12.1.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-12.1    #7904      +/-   ##
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+ Coverage         89.57%   89.65%   +0.07%     
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  Files               276      276              
  Lines             59660    59664       +4     
  Branches           7448     7448              
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+ Hits              53442    53489      +47     
+ Misses             4081     4038      -43     
  Partials           2137     2137