branch-2.1: [Bug](set) fix find null get wrong result on set operators #48001

[github-actions]

Cherry-picked from #48001

[Thearas]

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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Thearas]

run buildall

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 36.50% (9610/26327)
Line Coverage: 28.09% (79665/283613)
Region Coverage: 26.74% (40859/152779)
Branch Coverage: 23.50% (20708/88126)
Coverage Report: http://coverage.selectdb-in.cc/coverage/138a4546a58867d15965ecf8ac763084aa70ff98_138a4546a58867d15965ecf8ac763084aa70ff98/report/index.html