Correctly interpret None passed to query.get(); warn for empty PK values

Correctly interpret None passed to query.get(); warn for empty PK values

A warning is emitted if a primary key value is passed to :meth:`.Query.get`
that consists of None for all primary key column positions.   Previously,
passing a single None outside of a tuple would raise a ``TypeError`` and
passing a composite None (tuple of None values) would silently pass
through.   The fix now coerces the single None into a tuple where it is
handled consistently with the other None conditions.  Thanks to Lev
Izraelit for the help with this.

Fixes #4915
Closes: #4917
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4917
Pull-request-sha: b388343c7cabeecf8c779689b78e638c23f9af40

Change-Id: Ibc6c27ccf50dfd4adbf15b6dbd393115c30d44fb