Minor, fix an issue where we may fail to select a translation for mail.

src/applications/metamta/storage/PhabricatorMetaMTAMail.php

@@ -138,6 +138,11 @@ final class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
       }
       $return = $translation;
     }
+
+    if (!$return) {
+      $return = $default_translation;
+    }
+
     return $return;
   }