FIX: in EmailSettingsValidator, unset smtp authentication when there's no user and password

net-smtp 0.5.0 bails when authentication is set without username/password

followup-to: 7b8d60dc, 897be759

lib/tasks/emails.rake

@@ -84,7 +84,7 @@ task "emails:test", [:email] => [:environment] do |_, args|
       domain: smtp[:domain] || "localhost",
       username: smtp[:user_name],
       password: smtp[:password],
-      authentication: smtp[:authentication] || "plain",
+      authentication: smtp[:authentication],
     )
   rescue Exception => e
     if e.to_s.match(/execution expired/)