Fix missing None handling of Table.prefixes

Fix missing None handling of Table.prefixes

Fixed issue where passing ``None`` for the value of
:paramref:`_schema.Table.prefixes` would not store an empty list, but
rather the constant ``None``, which may be unexpected by third party
dialects. The issue is revealed by a usage in recent versions of Alembic
that are passing ``None`` for this value. Pull request courtesy Kai
Mueller.

Fixes: #6685
Closes: #6672
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6672
Pull-request-sha: b79aca0ee4011b244978b35fed4c687ffbe56dc9

Change-Id: I758641c6fbde6f2607d074fecea7efa6728aeea0