Fix 500 error caused by CODEOWNERS with no matches

Including a CODEOWNERS file with lines without any matching username or
e-mail regular expressions would cause an Error 500. Don't attempt a
database query if there is nothing to query.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10282

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

@@ -17,7 +17,11 @@ module Gitlab
     def users
       return User.none unless @text.present?
 
-      @users ||= User.from_union(union_relations)
+      relations = union_relations
+
+      return User.none unless relations.any?
+
+      @users ||= User.from_union(relations)
     end
 
     def usernames

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