### 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_
	ERR_FAIL_COND(!windows.has(p_window));
	WindowData &wd = windows[p_window];
	switch (p_flag) {
		case WINDOW_FLAG_RESIZE_DISABLED: {
			wd.resize_disabled = p_enabled;
			_update_size_hints(p_window);
			XFlush(x11_display);
		} break;
		case WINDOW_FLAG_BORDERLESS: {
			Hints hints;
			Atom property;
			hints.flags = 2;
			hints.decorations = p_enabled ? 0 : 1;
			property = XInternAtom(x11_display, "_MOTIF_WM_HINTS", True);
			if (property != None) {
				XChangeProperty(x11_display, wd.x11_window, property, property, 32, PropModeReplace, (unsigned char *)&hints, 5);
			}
			
			window_set_size(window_get_size(p_window), p_window);
			wd.borderless = p_enabled;
			_update_window_mouse_passthrough(p_window);
		} break;
		case WINDOW_FLAG_ALWAYS_ON_TOP: {
			ERR_FAIL_COND_MSG(wd.transient_parent != INVALID_WINDOW_ID, "Can't make a window transient if the 'on top' flag is active.");
			if (p_enabled && wd.fullscreen) {