[Hotfix] Handle null partition values in IcebergTableMaintainer to prevent expiration errors

[xxubai]

(cherry picked from commit d5a5daa765ea256b2be57063c6262addd7ad50c8)

Why are the changes needed?

Close #xxx.

Brief change log

• avoid NPE when expire data and try to expire data via file metrics

How was this patch tested?

• [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

• [ ] Add screenshots for manual tests if appropriate

• [x] Run test locally before making a pull request

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not documented)