### 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)

XGetWindowAttributes(x11_display, wd_parent.x11_window, &xwa);
		
		
		
		if (!wd_window.no_focus && !wd_window.is_popup && wd_window.focused) {
			if ((xwa.map_state == IsViewable) && !wd_parent.no_focus && !wd_window.is_popup) {
				XSetInputFocus(x11_display, wd_parent.x11_window, RevertToPointerRoot, CurrentTime);
			}
		}
	} else {
		ERR_FAIL_COND(!windows.has(p_parent));
		ERR_FAIL_COND_MSG(prev_parent != INVALID_WINDOW_ID, "Window already has a transient parent");
		WindowData &wd_parent = windows[p_parent];
		wd_window.transient_parent = p_parent;
		wd_parent.transient_children.insert(p_window);
		XSetTransientForHint(x11_display, wd_window.x11_window, wd_parent.x11_window);
	}
}
void DisplayServerX11::_update_size_hints(WindowID p_window) {
	WindowData &wd = windows[p_window];
	WindowMode window_mode = window_get_mode(p_window);
	XSizeHints *xsh = XAllocSizeHints();
	
	xsh->flags |= PPosition | PSize;
	xsh->x = wd.position.x;
	xsh->y = wd.position.y;
	xsh->width = wd.size.width;
	xsh->height = wd.size.height;
	if (window_mode == WINDOW_MODE_FULLSCREEN || window_mode == WINDOW_MODE_EXCLUSIVE_FULLSCREEN) {