### 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)

break;
			}
			if (fallback != nullptr) {
				cursor_img[i] = XcursorLibraryLoadImage(fallback, cursor_theme, cursor_size);
			}
		}
		if (cursor_img[i]) {
			cursors[i] = XcursorImageLoadCursor(x11_display, cursor_img[i]);
		} else {
			print_verbose("Failed loading custom cursor: " + String(cursor_file[i]));
		}
	}
	{
		
		
		Pixmap cursormask = XCreatePixmap(x11_display,
				RootWindow(x11_display, DefaultScreen(x11_display)), 1, 1, 1);
		
		XGCValues xgc;
		xgc.function = GXclear;
		GC gc = XCreateGC(x11_display, cursormask, GCFunction, &xgc);
		XFillRectangle(x11_display, cursormask, gc, 0, 0, 1, 1);
		
		XColor col = {};
		Cursor cursor = XCreatePixmapCursor(x11_display,
				cursormask, 
				cursormask, 
				&col, &col, 0, 0);
		XFreePixmap(x11_display, cursormask);
		XFreeGC(x11_display, gc);