### 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)

_THREAD_SAFE_METHOD_
	if (p_mode == mouse_mode) {
		return;
	}
	if (mouse_mode == MOUSE_MODE_CAPTURED || mouse_mode == MOUSE_MODE_CONFINED || mouse_mode == MOUSE_MODE_CONFINED_HIDDEN) {
		XUngrabPointer(x11_display, CurrentTime);
	}
	
	bool show_cursor = (p_mode == MOUSE_MODE_VISIBLE || p_mode == MOUSE_MODE_CONFINED);
	bool previously_shown = (mouse_mode == MOUSE_MODE_VISIBLE || mouse_mode == MOUSE_MODE_CONFINED);
	if (show_cursor && !previously_shown) {
		WindowID window_id = get_window_at_screen_position(mouse_get_position());
		if (window_id != INVALID_WINDOW_ID) {
			_send_window_event(windows[window_id], WINDOW_EVENT_MOUSE_ENTER);
		}
	}
	for (const KeyValue<WindowID, WindowData> &E : windows) {
		if (show_cursor) {
			XDefineCursor(x11_display, E.value.x11_window, cursors[current_cursor]); 
		} else {
			XDefineCursor(x11_display, E.value.x11_window, null_cursor);