Don't blow up when email has no References header

If an email doesn't match our incoming email patterns on the To header, we fall
back to the References header. If there was no References header, we'd raise an
exception, when we'd be better off acting as if it was empty.

changelogs/unreleased/dont-blow-up-when-email-has-no-references-header.yml

+---
+title: Gracefully handle failures for incoming emails which do not match on the To
+  header, and have no References header
+merge_request:
+author:

lib/gitlab/email/receiver.rb

@@ -70,6 +70,8 @@ module Gitlab
           # Handle emails from clients which append with commas,
           # example clients are Microsoft exchange and iOS app
           Gitlab::IncomingEmail.scan_fallback_references(references)
+        when nil
+          []
         end
       end

spec/lib/gitlab/email/receiver_spec.rb

@@ -7,9 +7,17 @@ describe Gitlab::Email::Receiver, lib: true do
   context "when we cannot find a capable handler" do
     let(:email_raw) { fixture_file('emails/valid_reply.eml').gsub(mail_key, "!!!") }
-    it "raises a UnknownIncomingEmail" do
+    it "raises an UnknownIncomingEmail error" do
       expect { receiver.execute }.to raise_error(Gitlab::Email::UnknownIncomingEmail)
     end
+
+    context "and the email contains no references header" do
+      let(:email_raw) { fixture_file("emails/auto_reply.eml").gsub(mail_key, "!!!") }
+
+      it "raises an UnknownIncomingEmail error" do
+        expect { receiver.execute }.to raise_error(Gitlab::Email::UnknownIncomingEmail)
+      end
+    end
   end
   context "when the email is blank" do