[bugfix] handle string type in TextConverter::write_column

[jacktengg]

Proposed changes

Issue Number: close #11255

Problem Summary:

Checklist(Required)

1. Type of your changes:
   • Improvement
   • Fix
   • Feature-WIP
   • Feature
   • Doc
   • Refator
   • Others:
2. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
3. Has unit tests been added:
   • Yes
   • No
   • No Need
4. Has document been added or modified:
   • Yes
   • No
   • No Need
5. Does it need to update dependencies:
   • Yes
   • No
6. Are there any changes that cannot be rolled back:
   • Yes
   • 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...