is_super_admin() should try to get missing user_id

[filosofo]

If its user_id argument is missing, is_super_admin() should assume we want the current user's ID and try to get that before returning false.

To clarify, instead of checking the global $current_user directly is_super_admin() should rely on the current user API as the functions used are pluggable and thus they might not be setting $current_user in predictable ways.

Gets my vote.

Changed patch to use ! empty instead of isset since $current_user can get set to false instead of being an object.

[12650] [12651]