ssd-titlebar: show fallback icon when no app_id is set

ssd-titlebar: show fallback icon when no app_id is set

Before this commit, nothing was rendered in window icon button if the
application doesn't set its app_id (e.g. nested kwin_wayland). This was my
oversight in dc474521.

This commit makes sure fallback icon (set via <theme><fallbackAppIcon>)
by removing ssd->state.app_id and the early-return in
ssd_update_window_icon(). This doesn't affect performance because
scaled_icon_buffer caches app_id and scaled_icon_buffer_set_app_id() does
nothing when a duplicated app_id is passed.