[Bugfix](load) fix be may core dump when load column mapping has function

[yangzhg]

Proposed changes

Issue Number: close #12507

Problem summary

fix be may core dump when load column mapping has function
this bug may be introduced by #12375

Checklist(Required)

1. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
2. Has unit tests been added:
   • Yes
   • No
   • No Need
3. Has document been added or modified:
   • Yes
   • No
   • No Need
4. Does it need to update dependencies:
   • Yes
   • No
5. Are there any changes that cannot be rolled back:
   • Yes (If Yes, please explain WHY)
   • No

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...

[Gabriel39]

LGTM.

Note: Actually, this bug is caused by processing nullable info incorrectly on row-based engine. And we don't decide to fix this on this engine obsoleted soon so just skip the logics here.

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.