Don't offer "specific project" on Special:OAuthConsumerRegistration/propose when no other projects are available

If there aren't any valid options to be entered for the "specific
project" 'other' box (besides duplicating the other two entries in the
dropdown), don't offer the 'other' box at all.

Bug: 73163
Change-Id: Ic9e729839879689042588de303e1c8a73030867b