Ignore empty incoming messages (Reply by email)

We should also update mail_room when https://github.com/tpitale/mail_room/pull/35 goes in, but that can wait a bit.

app/mailers/email_rejection_mailer.rb

@@ -3,6 +3,8 @@ class EmailRejectionMailer < BaseMailer
@reason = reason
@original_message = Mail::Message.new(original_raw)
+    return unless @original_message.from
+
headers = {
to: @original_message.from,
subject: "[Rejected] #{@original_message.subject}"