Handle case where InstanceState.obj returns None

Handle case where InstanceState.obj returns None

Fixed bug where a call ``InstanceState.obj()`` could return None when
synchronizing the instance states of the objects in the session in case
they become out of scope but are not yet finalized by the gc. This
case does not happen in cPython, but it may present itself in pypy.

The approach is to allow None to be gracefully handled by the
evaluator itself, ensuring it returns None in all cases when None
is passed in.

Fixes: #5631
Change-Id: I53d38fbea2e72b2e677c6e7f70bf075a58e58945