libweston/desktop: invoke weston_desktop_api.committed when empty buffer
Missing the committed signal notification of desktop surface on other shells when client committed a null buffer.
Shells may not handler correctly when missing a committed. Should call to weston_desktop_api.committed in this case.
Signed-off-by: Tran Ba Khang(MS/EMC31-XC) Khang.TranBa@vn.bosch.com