### Godot version

Godot Engine v4.0.stable.official.92bee43ad

### System information

Windows 10

### Issue description

The documentation states that passing ```null``` into the image parameter of ```set_custom_mouse_cursor``` will change it back to the system cursor, however I find that it removes the cursor's image entirely, rather than setting it to the system's cursor.


### Steps to reproduce

```var isimage = false
var theimage = load("res://final.png")

func _on_my_button_pressed():
    if isimage == false:
        Input.set_custom_mouse_cursor(theimage, Input.CURSOR_ARROW, Vector2(16,16))
        isimage = true
        
    else:
        Input.set_custom_mouse_cursor(null)
        isimage = false```

When the button is pressed, the cursor is meant to change to the new cursor. When it has been pressed a second time, it changes back to the default cursor. However, upon pressing it a second time, it simply disappears. The cursor becomes visible again when the cursor exits and re-enters the window.

### Minimal reproduction project

[null mouse.zip](https://github.com/godotengine/godot/files/10901969/null.mouse.zip)

if (ime_window_event) {
					break;
				}
				if (!mouse_mode_grab) {
					_send_window_event(windows[window_id], WINDOW_EVENT_MOUSE_ENTER);
				}
			} break;
			case FocusIn: {
				DEBUG_LOG_X11("[%u] FocusIn window=%lu (%u), mode='%u' \n", frame, event.xfocus.window, window_id, event.xfocus.mode);
				if (ime_window_event || (event.xfocus.detail == NotifyInferior)) {
					break;
				}
				WindowData &wd = windows[window_id];
				last_focused_window = window_id;
				wd.focused = true;
				
				static unsigned int focus_order = 0;
				wd.focus_order = ++focus_order;
				_send_window_event(wd, WINDOW_EVENT_FOCUS_IN);
				if (mouse_mode_grab) {
					
					for (const KeyValue<WindowID, WindowData> &E : windows) {
						if (mouse_mode == MOUSE_MODE_CONFINED) {
							XUndefineCursor(x11_display, E.value.x11_window);
						} else if (mouse_mode == MOUSE_MODE_CAPTURED || mouse_mode == MOUSE_MODE_CONFINED_HIDDEN) { 
							XDefineCursor(x11_display, E.value.x11_window, null_cursor);
						}
						XGrabPointer(
								x11_display, E.value.x11_window, True,
								ButtonPressMask | ButtonReleaseMask | PointerMotionMask,