Apply default robot policy for non-existing user pages

MediaWiki is sending HTTP 200 for every user page, this can result in
search engine indexing user pages with block hint. This is not happen,
when the page exists (even it is empty), so applying the default robot
policy also for non-existing user pages to avoid this.

Change-Id: Ia62b977b3488d6b002c7b129d9308b7887e4f42d