fix($state): allow null to be passed as 'params' param

$state.is('parent.child') returns true
$state.is('parent.child', null) returns false

http://plnkr.co/edit/YnXapCQVBuutf9dL7eX3?p=preview

This may cause problems if people wish to use the changes implemented by #636 since some will opt to ignore the second parameter using 'null' rather than 'undefined'.