Fix useless assertTrue

In glance/tests/functional/test_scrubber.py,
TestScrubber::test_scrubber_with_metadata_enc(), local var marker_uri
is always not None, so self.assertTrue(marker_uri is not None) don't
make sense. This patch init it with None.

Change-Id: Id2fcffe6ff67b6fa59247d42ab15ffb228dd622f
Merge "Fix useless assertTrue"