Fix get_table_comment return value

[andrasore-kodinfo]

No description provided.

[bchoudhary6415]

@andrasore-kodinfo
Thanks for raising pull request.
It will be great help if you can explain for which issue you have raised PR.
If you can share some sample repo regarding this issue.

Thanks

[andrasore-kodinfo]

@bchoudhary6415
This fixes apache/superset#24130.
Superset is calling get_table_comment while getting metadata for it's database tables. However, since ibm_db_sa is returning an SQLAlchemy cursor object instead of a string, the result cannot be serialized to JSON and the request crashes on Superset's backend.
I've fixed get_table_comment so it would return a string value instead.