Correct check whether the e-mail field was filled in Special:Userlogin/signup

Use a strict comparison with '' instead of empty(), otherwise this allows user
to give "0" as an e-mail address.

Also use strval() if in any case null or something like that would be passed.

Change-Id: Ide7d35c52a04d05b43ae9fd37f7586bb4a67d4ed
Merge "Correct check whether the e-mail field was filled in Special:Userlogin/signup"