Don't blow up when email has no References header

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