if in doubt, prefer unwrap_or_default()

if the past we had lots of crashes because of unexpected unwrap failures,
mostly related to string.
this commit avoids them eg. for string-conversions that may panic
eg. when encountering a null-byte or by logical programming errors
where an object is assumed to be set but is not under unexpected circumstances.

src/smtp.rs

@@ -81,7 +81,7 @@ impl Smtp {
             }
             let user = &lp.send_user;
 
-            lettre::smtp::authentication::Credentials::new(user.to_string(), access_token.unwrap())
+            lettre::smtp::authentication::Credentials::new(user.to_string(), access_token.unwrap_or_default())
         } else {
             // plain
             let user = lp.send_user.clone();