[fix](join)fix outer join bug when a subquery as nullable side

[starocean999]

Proposed changes

Issue Number: close #xxx

Problem summary

When the subquery's root node is a agg node, the output slot may come from the child outer join node's nullable side. This nullable info is lost when creating the output tuple desc for agg node. This pr fixed this problem.

Checklist(Required)

1. Does it affect the original behavior:
   • [ ] Yes
   • [x] No
   • [ ] I don't know
2. Has unit tests been added:
   • [ ] Yes
   • [x] No
   • [ ] No Need
3. Has document been added or modified:
   • [ ] Yes
   • [x] No
   • [ ] No Need
4. Does it need to update dependencies:
   • [ ] Yes
   • [x] No
5. Are there any changes that cannot be rolled back:
   • [ ] Yes (If Yes, please explain WHY)
   • [x] No

Further comments

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