### 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)

locale = getenv("LC_CTYPE");
			}
			if (!locale || !*locale) {
				locale = getenv("LANG");
			}
			if (!locale || !*locale) {
				locale = "C";
			}
			dead_tbl = xkb_compose_table_new_from_locale(xkb_ctx, locale, XKB_COMPOSE_COMPILE_NO_FLAGS);
		}
	}
#endif
	Input::get_singleton()->set_event_dispatch_function(_dispatch_input_events);
	r_error = OK;
	{
		if (!XcursorImageCreate || !XcursorImageLoadCursor || !XcursorImageDestroy || !XcursorGetDefaultSize || !XcursorGetTheme || !XcursorLibraryLoadImage) {
			
			ERR_PRINT("Unsupported Xcursor library version.");
			r_error = ERR_UNAVAILABLE;
			return;
		}
	}
	for (int i = 0; i < CURSOR_MAX; i++) {
		cursors[i] = None;
		cursor_img[i] = nullptr;
	}
	XInitThreads(); 
	
	x11_display = XOpenDisplay(nullptr);
	if (!x11_display) {