Reflect table can reflect table with no columns

Reflect table can reflect table with no columns

The :meth:`_engine.Inspector.reflect_table` method now supports reflecting
tables that do not have user defined columns. This allows
:meth:`_schema.MetaData.reflect` to properly complete reflection on
databases that contain such tables.   Currently, only PostgreSQL is known
to support such a construct among the common database backends.

Fixes: #3247
Closes: #7118
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7118
Pull-request-sha: cb8ce01957e9a1453290a7c2728af8c60ef55fa1

Change-Id: I906cebe17d13554d79086b92f3e1e51ffba3e818