Fix checks to pass or display "return to" links in Special:UserLogin.

Fix checks to pass or display "return to" links in Special:UserLogin.

Currently it is not possible to have a "return to" link to the page
"0" because "empty( '0' )" is true.

- Groupped mReturnTo and mReturnToQuery definitions for better
  consistency.
- Changed default value of those members from null to '' when they
  are not provided so that it's easier to make strict comparison
- Changed empty() checks to strict comparison with '' to correctly
  display the links in the case mentionned above

Change-Id: I2714ba1c5654f6ea6ddb195eef0c7464893224e0