### 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)

KeyMappingX11::initialize();
#ifdef SOWRAP_ENABLED
#ifdef DEBUG_ENABLED
	int dylibloader_verbose = 1;
#else
	int dylibloader_verbose = 0;
#endif
	if (initialize_xlib(dylibloader_verbose) != 0) {
		r_error = ERR_UNAVAILABLE;
		ERR_FAIL_MSG("Can't load Xlib dynamically.");
	}
	if (initialize_xcursor(dylibloader_verbose) != 0) {
		r_error = ERR_UNAVAILABLE;
		ERR_FAIL_MSG("Can't load XCursor dynamically.");
	}
#ifdef XKB_ENABLED
	bool xkb_loaded = (initialize_xkbcommon(dylibloader_verbose) == 0);
	xkb_loaded_v05p = xkb_loaded;
	if (!xkb_context_new || !xkb_compose_table_new_from_locale || !xkb_compose_table_unref || !xkb_context_unref || !xkb_compose_state_feed || !xkb_compose_state_unref || !xkb_compose_state_new || !xkb_compose_state_get_status || !xkb_compose_state_get_utf8) {
		xkb_loaded_v05p = false;
		print_verbose("Detected XKBcommon library version older than 0.5, dead key composition and Unicode key labels disabled.");
	}
	xkb_loaded_v08p = xkb_loaded;
	if (!xkb_keysym_to_utf32 || !xkb_keysym_to_upper) {
		xkb_loaded_v08p = false;
		print_verbose("Detected XKBcommon library version older than 0.8, Unicode key labels disabled.");
	}
#endif
	if (initialize_xext(dylibloader_verbose) != 0) {