This pr do two things:
1. fix:
It use `column[0]` to judge class type in JdbcExecutor, but column[0] may be null !
2. Enhencement
In the original logic, all fields in jdbc catalog table will be set Nullable.
However, it is inefficient for nullable fields. Actually, we can know if the fields in data source table
is nullable through jdbc. So we can set the corresponding fields in Doris jdbc catalog to nullable or not.
c2cc75d741