[release-20.0] Handle Nullability for Columns from Outer Tables (#16174)

[vitess-bot]

Description

This is a backport of #16174

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

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• There should be a comment at the top of each new or modified test to explain what the test does.

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Backward compatibility

• Protobuf changes should be wire-compatible.
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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.58%. Comparing base (ba19342) to head (00efe57).
Report is 3 commits behind head on release-20.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16187      +/-   ##
================================================
+ Coverage         68.54%   68.58%   +0.04%     
================================================
  Files              1541     1541              
  Lines            197628   197915     +287     
================================================
+ Hits             135471   135748     +277     
- Misses            62157    62167      +10     

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