fix AttributeError in twitter username validator
Ran into this while testing locally; an alternate fix would be to make twitter_username required, but as it's not necessary for operation of Yak-Bak, I think it's fine to be optional.
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Ran into this while testing locally; an alternate fix would be to make twitter_username required, but as it's not necessary for operation of Yak-Bak, I think it's fine to be optional.