Fix a warning about null sender in QObject::connect()

The original assumption was that once PointerInputRedirection starts pushing cursor sources to cursors, they cannot be null. In many cases, it is true, but the tests are a bit special as many of them lack wl_pointer.set_cursor requests, so it's possible to set a null source. As an example, when the pointer moves from the decoration to the surface. On the other hand, it also does make sense to allow having no source connected.