wm: revoke curr focus if new focus is undefined

This allows the window layouter to ensure that input entered after
switching to an empty screen won't be routed to the old focused but
no longer visible window.

Issue genodelabs#5390

repos/gems/src/server/wm/main.cc

@@ -79,10 +79,14 @@ struct Wm::Main : Pointer::Tracker, Gui::Session_component::Action
 	{
 		_gui_root.revoke_exclusive_input();
 		_focus_rom.update();
+		bool defined = false;
 		_focus_rom.xml().with_optional_sub_node("window", [&] (Xml_node const &window) {
 			_with_win_id_from_xml(window, [&] (Window_registry::Id id) {
 				_gui_root.with_gui_session(id, [&] (Capability<Gui::Session> cap) {
-					_focus_gui_session.focus(cap); }); }); });
+					_focus_gui_session.focus(cap);
+					defined = true; }); }); });
+		if (!defined)
+			_focus_gui_session.focus({ });
 	}
 
 	Signal_handler<Main> _focus_handler {