OAK-1917: FileNotFoundException during TarMK GC

The "graph != null" condition from OAK-1780 should only be used to force
the rewriting of an old tar file when that tar file still contains some
referenceable content, not when the entire file can be removed.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1605670 13f79535-47bb-0310-9956-ffa450edef68