Fix confusing error when webfinger request returns empty document (#16986)
Fix confusing error when webfinger request returns empty document (#16986)

For some reason, some misconfigured servers return an empty document when
queried over webfinger. Since an empty document does not lead to a parse
error, the error is not caught properly and triggers uncaught exceptions
later on.

This PR fixes that by immediately erroring out with `Webfinger::Error` on
getting an empty response.
koba-lab/mastodon,koba-lab/mastodon,koba-lab/mastodon,koba-lab/mastodon