default Enum name to None and don't remove given name

default Enum name to None and don't remove given name

Fixed issue in support for the :class:`.Enum` datatype in the
:paramref:`_orm.registry.type_annotation_map` first added as part of
:ticket:`8859` where using a custom :class:`.Enum` with fixed configuration
in the map would fail to transfer the :paramref:`.Enum.name` parameter,
which among other issues would prevent PostgreSQL enums from working if the
enum values were passed as individual values.  Logic has been updated so
that "name" is transferred over, but also that the default :class:`.Enum`
which is against the plain Python `enum.Enum` class or other "empty" enum
won't set a hardcoded name of ``"enum"`` either.

Fixes: #9963
Change-Id: I36526dcb5443579fcd604b0c02bd1f02ca85a977