### 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)

m.window = xdnd_source_window;
					m.message_type = xdnd_finished;
					m.format = 32;
					m.data.l[0] = windows[window_id].x11_window;
					m.data.l[1] = 1;
					m.data.l[2] = xdnd_action_copy; 
					XSendEvent(x11_display, xdnd_source_window, False, NoEventMask, (XEvent *)&m);
				}
				break;
			case ClientMessage:
				if (ime_window_event) {
					break;
				}
				if ((unsigned int)event.xclient.data.l[0] == (unsigned int)wm_delete) {
					_send_window_event(windows[window_id], WINDOW_EVENT_CLOSE_REQUEST);
				}
				else if ((unsigned int)event.xclient.message_type == (unsigned int)xdnd_enter) {
					
					xdnd_version = (event.xclient.data.l[1] >> 24);
					Window source = event.xclient.data.l[0];
					bool more_than_3 = event.xclient.data.l[1] & 1;
					if (more_than_3) {
						Property p = _read_property(x11_display, source, XInternAtom(x11_display, "XdndTypeList", False));
						requested = pick_target_from_list(x11_display, (Atom *)p.data, p.nitems);
						XFree(p.data);
					} else {
						requested = pick_target_from_atoms(x11_display, event.xclient.data.l[2], event.xclient.data.l[3], event.xclient.data.l[4]);
					}
				} else if ((unsigned int)event.xclient.message_type == (unsigned int)xdnd_position) {