add option to create scalar object on none attribute set

add option to create scalar object on none attribute set

Added new option to :func:`.association_proxy`
:paramref:`.association_proxy.create_on_none_assignment`; when an
association proxy which refers to a scalar relationship is assigned the
value ``None``, and the referenced object is not present, a new object is
created via the creator.  This was apparently an undefined behavior in the
1.2 series that was silently removed.

Fixes: #10013
Change-Id: I3aae484b8cf5218588b1db63e691cd86214fbbad