git-annex S3 special remotes have no url key; running
`git config --get-all remote.<name>.url` on them exits 1, which made
tea hard-fail in Config() on any repo that has such a remote. Treat a
missing key (exit 1 with empty or 'No such section or key' stderr) as an
empty URL list, mirroring the existing branch-section handling.
magerbeton