Merge branch 'sh-fix-blank-codeowners-ce' into 'master'

Fix 500 error caused by CODEOWNERS with no matches

Closes gitlab-ee#10282

See merge request gitlab-org/gitlab-ce!26072

changelogs/unreleased/sh-fix-blank-codeowners-ce.yml

+---
+title: Fix 500 error caused by CODEOWNERS with no matches
+merge_request: 26072
+author:
+type: fixed

lib/gitlab/user_extractor.rb

@@ -16,6 +16,7 @@ module Gitlab
     def users
       return User.none unless @text.present?
+      return User.none if references.empty?
       @users ||= User.from_union(union_relations)
     end

spec/lib/gitlab/user_extractor_spec.rb

@@ -48,6 +48,14 @@ describe Gitlab::UserExtractor do
     it 'includes all mentioned usernames' do
       expect(extractor.matches[:usernames]).to contain_exactly('user-1', 'user-2', 'user-4')
     end
+
+    context 'input has no matching e-mail or usernames' do
+      it 'returns an empty list of users' do
+        extractor = described_class.new('My test')
+
+        expect(extractor.users).to be_empty
+      end
+    end
   end
   describe '#references' do