tagcache: Don't try to look up "deleted" filename entries
When a file is deleted from the database, the first character of that entry
is changed to \0. However the entry's length is not updated,
because the entry is still using space in the index.
When checking for deleted files, we were only keying on "length == 0"
instead of also checking to see if the entry itself starts with \0.
Change-Id: Ic00a0b29c3857c597ee5ff6ec01a5f4fd9633447