### 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)

return current_cursor;
}
void DisplayServerX11::cursor_set_custom_image(const Ref<Resource> &p_cursor, CursorShape p_shape, const Vector2 &p_hotspot) {
	_THREAD_SAFE_METHOD_
	ERR_FAIL_INDEX(p_shape, CURSOR_MAX);
	if (p_cursor.is_valid()) {
		HashMap<CursorShape, Vector<Variant>>::Iterator cursor_c = cursors_cache.find(p_shape);
		if (cursor_c) {
			if (cursor_c->value[0] == p_cursor && cursor_c->value[1] == p_hotspot) {
				cursor_set_shape(p_shape);
				return;
			}
			cursors_cache.erase(p_shape);
		}
		Ref<Texture2D> texture = p_cursor;
		ERR_FAIL_COND(!texture.is_valid());
		Ref<AtlasTexture> atlas_texture = p_cursor;
		Size2i texture_size;
		Rect2i atlas_rect;
		if (atlas_texture.is_valid()) {
			texture = atlas_texture->get_atlas();
			atlas_rect.size.width = texture->get_width();
			atlas_rect.size.height = texture->get_height();
			atlas_rect.position.x = atlas_texture->get_region().position.x;
			atlas_rect.position.y = atlas_texture->get_region().position.y;
			texture_size.width = atlas_texture->get_region().size.x;
			texture_size.height = atlas_texture->get_region().size.y;
		} else {