[Rspamd] meta_exporter: return false if not matched

[Compose] Update Dovecot image

data/conf/rspamd/local.d/metadata_exporter.conf

@@ -20,7 +20,7 @@ return function(task)
   if ratelimited then
     return true
   end
-  return
+  return false
 end
 EOD;
 }