InstanceState default path is RootRegistry, not tuple

InstanceState default path is RootRegistry, not tuple

Fixed regression caused in 1.3.13 by :ticket:`5056` where a refactor of the
ORM path registry system made it such that a path could no longer be
compared to an empty tuple, which can occur in a particular kind of joined
eager loading path.   The "empty tuple" use case has been resolved so that
the path registry is compared to a path registry in all cases;  the
:class:`.PathRegistry` object itself now implements ``__eq__()`` and
``__ne__()`` methods which will take place for all equality comparisons and
continue to succeed in the not anticipated case that a non-
:class:`.PathRegistry` object is compared, while emitting a warning that
this object should not be the subject of the comparison.

Fixes: #5110
Change-Id: I6cab6cd771c131d12b17939b369212f12c6bee16