The ``cached_result`` attribute of ``FixtureDef`` is now set to ``None`` when
the result is unavailable, instead of being deleted.

If your plugin performs checks like ``hasattr(fixturedef, 'cached_result')``,
for example in a ``pytest_fixture_post_finalizer`` hook implementation, replace
it with ``fixturedef.cached_result is not None``. If you ``del`` the attribute,
set it to ``None`` instead.