Ensure userlogin.js checks for wgMFLoginHandshakeUrl before handshaking

Without this, if you leave wgMFLoginHandhakeUrl == false, this will
attempt to redirect you to /null!

Change-Id: I92e2ce8fe21682fcd6022f1f0f4296e5fb6c1218
Merge "Ensure userlogin.js checks for wgMFLoginHandshakeUrl before handshaking"