Update SmtpMailAttributes.cs

Checking `From.Address` will cause an error if `From` has never been set and is still null. Trying to explicitly create a new `MailAddress` object in the `From` field to satisfy this check by passing in null `MailAddress(null)` or an empty string `MailAddress(string.Empty)` will both cause errors as well, there should be no condition under which From.Address is ever null or empty.