dxdiagn: Calling GetChildContainer with an empty string on a leaf container returns the object itself.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=38014Signed-off-by: Gijs Vermeulen <gijsvrm@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>