### 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)

XWindowAttributes xwa;
			XSync(x11_display, False);
			XGetWindowAttributes(x11_display, wd.x11_window, &xwa);
			ERR_FAIL_COND_MSG(p_window == MAIN_WINDOW_ID, "Main window can't be popup.");
			ERR_FAIL_COND_MSG((xwa.map_state == IsViewable) && (wd.is_popup != p_enabled), "Popup flag can't changed while window is opened.");
			wd.is_popup = p_enabled;
		} break;
		default: {
		}
	}
}
bool DisplayServerX11::window_get_flag(WindowFlags p_flag, WindowID p_window) const {
	_THREAD_SAFE_METHOD_
	ERR_FAIL_COND_V(!windows.has(p_window), false);
	const WindowData &wd = windows[p_window];
	switch (p_flag) {
		case WINDOW_FLAG_RESIZE_DISABLED: {
			return wd.resize_disabled;
		} break;
		case WINDOW_FLAG_BORDERLESS: {
			bool borderless = wd.borderless;
			Atom prop = XInternAtom(x11_display, "_MOTIF_WM_HINTS", True);
			if (prop != None) {
				Atom type;
				int format;
				unsigned long len;
				unsigned long remaining;
				unsigned char *data = nullptr;
				if (XGetWindowProperty(x11_display, wd.x11_window, prop, 0, sizeof(Hints), False, AnyPropertyType, &type, &format, &len, &remaining, &data) == Success) {
					if (data && (format == 32) && (len >= 5)) {