### 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)

}
	xim = XOpenIM(x11_display, nullptr, nullptr, nullptr);
	if (xim == nullptr) {
		WARN_PRINT("XOpenIM failed");
		xim_style = 0L;
	} else {
		::XIMCallback im_destroy_callback;
		im_destroy_callback.client_data = (::XPointer)(this);
		im_destroy_callback.callback = (::XIMProc)(_xim_destroy_callback);
		if (XSetIMValues(xim, XNDestroyCallback, &im_destroy_callback,
					nullptr) != nullptr) {
			WARN_PRINT("Error setting XIM destroy callback");
		}
		::XIMStyles *xim_styles = nullptr;
		xim_style = 0L;
		char *imvalret = XGetIMValues(xim, XNQueryInputStyle, &xim_styles, nullptr);
		if (imvalret != nullptr || xim_styles == nullptr) {
			fprintf(stderr, "Input method doesn't support any styles\n");
		}
		if (xim_styles) {
			xim_style = 0L;
			for (int i = 0; i < xim_styles->count_styles; i++) {
				const ::XIMStyle &style = xim_styles->supported_styles[i];
				if (!_is_xim_style_supported(style)) {
					continue;
				}
				xim_style = _get_best_xim_style(xim_style, style);
			}
			XFree(xim_styles);
		}