[fix](select join) Make selected slotRef nullable when slotRef is from nullable tuple in outer join sql block

[caiconghui]

Proposed changes

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Checklist

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• I have created an issue on (Fix [Bug] Execute left out join sql with group_concat function will cause be core #7288) and described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
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• Any dependent changes have been merged

Further comments

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[morningman]

LGTM cc @EmmyMiao87

[EmmyMiao87]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.