Add FakeApplicationSettings#commit_email_hostname

Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/54953-fix-commit_email_hostname-accessor-in-fake_application_settings.yml

+---
+title: Fix a 500 error that could occur until all migrations are done
+merge_request: 23939
+author:
+type: fixed

lib/gitlab/fake_application_settings.rb

@@ -37,5 +37,9 @@ module Gitlab
     def pick_repository_storage
       repository_storages.sample
     end
+
+    def commit_email_hostname
+      super.presence || ApplicationSetting.default_commit_email_hostname
+    end
   end
 end