Consider merge key with (None, ) as non-persistent

Consider merge key with (None, ) as non-persistent

Fixed bug in :meth:`.Session.merge` where objects in a collection that had
the primary key attribute set to ``None`` for a key that is  typically
autoincrementing would be considered to be a database-persisted key for
part of the internal deduplication process, causing only one object to
actually be inserted in the database.

Change-Id: I0a6e00043be0b2979cda33740e1be3b430ecf8c7
Fixes: #4056
(cherry picked from commit 5243341ed886e10a0d3f7fef8ae3d071e0ffdcf0)