[432]fixed nullable mismatch in coalesce

[lgbo-ustc]

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• Bug Fix

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

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Documentation entry for user-facing changes

• [ ] Documentation is written (mandatory for new features)

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closed #432

[kyligence-git]

Can one of the admins verify this patch?

[lgbo-ustc]

test this please with 1353