Fix bug#10810.

When login OTRS, output a following message in error log of WWW server.  

 [Fri Oct 17 10:31:49 2014] index.pl: Use of uninitialized value
$User{"UserAuthBackend"} in string eq at
/opt/otrs/bin/cgi-bin/../../Kernel/System/User.pm line 1114.

The PR fixes it by using `defined' instead of `exists'.