Dont send an empty challenge when no initial response in anonymous authentication

[albertony]

The trace information in anonymous authentication is optional, so I don't think the server should send an empty challenge back if it is not specified in optional initial response argument to the AUTH command in SMTP for instance.

This is related to emersion/go-smtp#258

[emersion]

There is a subtle detail in the go-sasl API here: response = nil means no initial response, and response = []byte{} (a non-nil empty slice) means that the initial response was sent but was empty. (See the doc comment for Server.)

I think the current code is correct.

(We should definitely get rid of that footgun though…)

[albertony]

Aha, I see. I missed that distinction. So I will update the go-smtp pr accordingly, and hopefully fix the issue entirely with that. This can therefore be closed, unless you meant there was some other improvement that should be done..?
Thanks.

[emersion]

I've opened #28 for the footgun.