dir: Return empty array instead of NULL while querying related-refs
dir: Return empty array instead of NULL while querying related-refs

Initialize the related-refs array with empty GPtrArray so that if
the remote has 'url= ' (for e.g., in case of flatpak bundle's remotes),
a empty array is returned instead of NULL.

(NULL mostly implies a operation has failed and error is set)

Also, this syncs the implementation of `if (*url == 0)` with
that of  flatak_dir_find_remote_related_for_metadata function.

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak