efl_ui_focus_object: Don't pass NULL to child_focus_set

efl_ui_focus_object: Don't pass NULL to child_focus_set

When starting elementary_test, efl_ui_focus_object_child_focus_set is
getting passed NULL on startup because elm_test window has no parent.
This makes it difficult to debug and fix T7030 issues. In order to fix
the issue, this patch checks for a valid parent before calling
efl_ui_focus_object_child_focus_set so that it no longer gets passed
NULL on elm_test startup and we can set breakpoints on eo.c:637
without getting interrupted at startup.

ref T7030

Reviewed-by: Marcel Hollerbach <marcel-hollerbach@t-online.de>
Differential Revision: https://phab.enlightenment.org/D7677