BUG: Export of Empty Authentication Key Can Occur

When the only enabled node on my Node Manager was a Gordian Server that had since been rebuilt, I ended up with a Tor V3 Authentication Public Key of `descriptor:x25519:` with no actual token.

Reproduce the case where the only enabled Node Manager node is a rebuilt Gordian Server, then search the iOS project for generation or export of the Tor V3 Authentication Public Key and the `descriptor:x25519:` prefix. Done means the wallet cannot export an authentication key with an empty token, with the rebuilt-server scenario covered by a regression test.

Written by the indexing model from the issue text.