Fix boolean check in new path comparison logic

Fix boolean check in new path comparison logic

Fixed regression where a new error message that was supposed to raise when
attempting to link a relationship option to an AliasedClass without using
:meth:`.PropComparator.of_type` would instead raise an ``AttributeError``.
Note that in 1.3, it is no longer valid to create an option path from a
plain mapper relationship to an :class:`.AliasedClass` without using
:meth:`.PropComparator.of_type`.

Fixes: #4566
Change-Id: Ic547a1c8408e41aec66ef9644aac7f76f50dd064