Fix SMTP config regression when not using authentication

[dsander]

user_name and password need to be set to nil when no authentication should be performed

Fixes #1657

[cantino]

LGTM. It might be nice to also add a magic "none" value for SMTP_AUTHENTICATION that makes it nil as well, since @virtadpt said he needed to set it to SMTP_AUTHENTICATION= in your .env to make this work?

[dsander]

I don't think that is needed, we defaulted to plain before the config change, Net::SMTP is only running the authentication when a user is passed.

[cantino]

LGTM then

[dsander]

Thanks!