BUG: pandas 3.0.0, pd.merge left join with 3 keys incorrectly matches a NaN key with a non-NaN value. (issue#64060)

[kjmin622]

… a NaN key with a non-NaN value. #64060

• closes BUG: pandas 3.0.0 and pyarrow 21.0.0, pd.merge left join with 3 keys incorrectly matches a NaN key with a non-NaN value. #64060 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• I have reviewed and followed all the contribution guidelines
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.