[release-20.0-rc] 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

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Backward compatibility

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.50%. Comparing base (4865893) to head (9c28e79).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release-20.0-rc   #16186      +/-   ##
===================================================
+ Coverage            68.47%   68.50%   +0.02%     
===================================================
  Files                 1541     1541              
  Lines               197614   197625      +11     
===================================================
+ Hits                135314   135376      +62     
+ Misses               62300    62249      -51     

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