GdkWaylandWindow: window_update_scale: Don't reset scale to 1 with no outputs
When there is
on_monitors_changed called on GdkWindow which doesn't have
any outputs on its
impl->display_server.outputs list, the scale was being
reset to 1, causing GtkPopovers to set their scale to 1 and having blurry
appearance on hidpi screens.
Fixes #7 (closed)