[migration] don't attempt to carry over values when renaming a computed relation
Without the fix in migractions.py, the introduced test crashes with an
operational error because of an attempt to write to a non-existing
relation table (computed relations are not materialized).
Closes #6304946
consider .do_fti flag in index_entity method
as this method may be used from outsite the class (eg in UpdateFTIHook), this is
the proper way to consider it anyway.
Closes #6206636