Make it clear that chaining up to the default implementation is optional if subclasses do not want preallocation. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6328>
And fix memory leaks/null pointer dereferences in GstUri in error cases. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3194>