ensure implicit_returning is checked on a Table instance

ensure implicit_returning is checked on a Table instance

Fixed regression where flushing a mapped class that's mapped against a
subquery, such as a direct mapping or some forms of concrete table
inheritance, would fail if the :paramref:`_orm.Mapper.eager_defaults`
parameter were used.

mapper.local_table can still be a non-table instance.
correct the check first added in 466ed5b53a3.

Fixes: #8812
Change-Id: I1bb2b83c31b910fbd96fcd3ac43e789b657aebf7