return empty MappedColumn() at early pep-593 step

return empty MappedColumn() at early pep-593 step

Fixed issue in new ORM Annotated Declarative where using a
:class:`_schema.ForeignKey` (or other column-level constraint) inside of
:func:`_orm.mapped_column` which is then copied out to models via pep-593
``Annotated`` would apply duplicates of each constraint to the
:class:`_schema.Column` as produced in the target :class:`_schema.Table`,
leading to incorrect CREATE TABLE DDL as well as migration directives under
Alembic.

Fixes: #9766
Change-Id: I8a3b2716bf393d1d2b5894f9f72b45fa59df1e08