elementary: Avoid passing NULL in some win focus methods
This one is rather harmless, as the NULL returned by this fuction is handled
correctly in all places, but this commit removes the runtime warning.
Test Plan: After this commit the warning disappears. Got the warning with win_example.
Reviewers: zmike, bu5hm4n, devilhorns
Reviewed By: devilhorns
Subscribers: netstar, cedric, committers
Maniphest Tasks: T7030
Differential Revision: https://phab.enlightenment.org/D6538