[bugfix](topn) fix topn do not work because Field == Null always return true

[xiaokang]

Proposed changes

Issue Number: close #xxx

if (!new_top.is_null() && new_top != old_top) is always false since old_top is Null when init and Field == Null always return true.

We add old_top.is_null() check first to avoid the problem and then issue more carefull discussion about Field == Null semantics.

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[xiaokang]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[hello-stephen]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 48.33 seconds
stream load tsv: 532 seconds loaded 74807831229 Bytes, about 134 MB/s
stream load json: 20 seconds loaded 2358488459 Bytes, about 112 MB/s
stream load orc: 64 seconds loaded 1101869774 Bytes, about 16 MB/s
stream load parquet: 31 seconds loaded 861443392 Bytes, about 26 MB/s
insert into select: 28.6 seconds inserted 10000000 Rows, about 349K ops/s
storage size: 17162106160 Bytes

[eldenmoon]

LGTM