Unable to authenticate Reddit using CLI tool when not passing a username in the commandline parameters

I've been trying to prove Reddit. It works, but not when only running `keybase prove reddit`. When I enter a username in whatever form, it just asks again.

```
hillebrand@T420s:~/Downloads$ keybase prove reddit
You already have a proof for [...] @reddit; overwrite? [y/N] y # just to prove this bug
Your username on Reddit: [...] # Entering my username...
Your username on Reddit: /u/[...] # Entering it in another format...
Your username on Reddit: @[...] # And another one....
Your username on Reddit: ▶ ERROR input canceled # Until the user just gives up.
hillebrand@T420s:~/Downloads$ keybase prove reddit [...]
You already have a proof for [...]@reddit; overwrite? [y/N] y
Please click on the following link to post to Reddit:

[...]
Check Reddit now? [Y/n] n # Don't want to authenticate again
▶ ERROR Proof wasn't available; we'll keep trying
```

Reproduce `keybase prove reddit` without a username, then compare it with the form that includes a username. Investigate why the interactive prompt rejects the entered username and verify that the prompt accepts the documented formats and that the Reddit proof flow completes.

Written by the indexing model from the issue text.