support configurable None behavior for composites

support configurable None behavior for composites

Added new parameter :paramref:`_orm.composite.return_none_on` to
:func:`_orm.composite`, which allows control over if and when this
composite attribute should resolve to ``None`` when queried or retrieved
from the object directly.  By default, a composite object is always present
on the attribute, including for a pending object which is a behavioral
change since 2.0.   When :paramref:`_orm.composite.return_none_on` is
specified, a callable is passed that returns True or False to indicate if
the given arguments indicate the composite should be returned as None. This
parameter may also be set automatically when ORM Annotated Declarative is
used; if the annotation is given as ``Mapped[SomeClass|None]``, a
:paramref:`_orm.composite.return_none_on` rule is applied that will return
``None`` if all contained columns are themselves ``None``.

Fixes: #12570
Change-Id: Iab01ac46065689da1332c9f80dedbc7eb0f5380b