Handle meta_screen_get_monitor_for_point() returning NULL

On the X11 backend we don't track the pointer position in
priv->current_x/y which remain set to zero. That means we never set
the clutter stage cursor if point 0,0 isn't covered by any monitor
since we return early.

Commit 4bebc5e5 introduced this to
avoid crashing on the prepare-at handlers when the cursor position
doesn't fall inside any monitor area but we can handle that higher up
in the stack. In that case, the sprite's scale doesn't matter since
the cursor won't be shown anyway so we can skip setting it.

https://bugzilla.gnome.org/show_bug.cgi?id=763159