Users can implement the smtp.Auth interface and return zero bytes in
the "toServer []byte" return value from the Auth.Start method. People
apparently do this to implement the SMTP "LOGIN" method.

But we were then sending "AUTH LOGIN \r\n" to the server, which some
servers apparently choke on. So, trim it when the toServer value is
empty.

Fixes #17794

Change-Id: I83662dba9e0f61b1c5000396c096cf7110f78361
Reviewed-on: https://go-review.googlesource.com/33143
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>