[fix](functions) fix now(null) crash

[zclllyybb]

Proposed changes

Issue Number: close #xxx

before: BE crash
now:

mysql [test]>select now(null);
+-----------+
| now(NULL) |
+-----------+
| NULL      |
+-----------+
1 row in set (0.06 sec)

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...

[zclllyybb]

run buildall

[github-actions]

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

[hello-stephen]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 44.91 seconds
stream load tsv: 508 seconds loaded 74807831229 Bytes, about 140 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: 29.1 seconds inserted 10000000 Rows, about 343K ops/s
storage size: 17162671190 Bytes

[zclllyybb]

run buildall

[github-actions]

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

[hello-stephen]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 44.4 seconds
stream load tsv: 507 seconds loaded 74807831229 Bytes, about 140 MB/s
stream load json: 20 seconds loaded 2358488459 Bytes, about 112 MB/s
stream load orc: 65 seconds loaded 1101869774 Bytes, about 16 MB/s
stream load parquet: 31 seconds loaded 861443392 Bytes, about 26 MB/s
insert into select: 29.6 seconds inserted 10000000 Rows, about 337K ops/s
storage size: 17162702545 Bytes

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.