Only put returnto parameter if needed on the from-http redirect in Special:Userlogin

The default value of the $mReturnTo and $mReturnToQuery member variables is '',
even if they were not passed. This means that they would always be present
in the redirected URL when accessing Special:Userlogin from HTTP and HTTPS
is available.

Now the special case '' is changed to null so that they don't appear anymore
if not needed.

Change-Id: I49a085feaa49d4b2954ff280db241234d2d46258
Merge "Only put returnto parameter if needed on the from-http redirect in Special:Userlogin"