Fix NPE in DiffServlet for missing path

The isFile method added by Ib11706aa did not handle the case where
TreeWalk.forPath returns null, as it does for a missing path.

This affected any diff URL with a non-empty, nonexistent path, e.g.:
  /project/+diff/master/notfound

Change-Id: I5c99be0cbdc99174c4a21b4deae6ac5b5bbd13fd