snapcraft list-keys doesn't show the non-registered key when no key is registered

[ethan.hsieh]

Here are the reproduction steps:

$ snapcraft create-key test

We're supposed to get following result
$ snapcraft list-keys
    Name SHA3-384 fingerprint
- test <fingerprint> (not registered)

But, list-keys returns no keys
$ snapcraft list-keys
No keys have been registered. See 'snapcraft register-key --help' to register a key.

Need to register the first key.
$ snapcraft register-key test
$ snapcraft list-keys
    Name SHA3-384 fingerprint
* test <fingerprint>

After the first key is registered, list-keys can show the non-registered key.
$ snapcraft create-key serial
$ snapcraft list-keys
    Name SHA3-384 fingerprint
* test <fingerprint>
- serial <fingerprint> (not registered)

---
$ snap list
Name Version Rev Tracking Publisher Notes
snapcraft 3.8 3440 stable canonical* classic