pkg/typify: empty messages are OtherError (#1531)

Messages with nothing in them are considered OtherError, they can not
serve any purpose for normal clients (i.e. dyn update or notifies might
have a use for them).

Also update a test in the cache plugin, so that we explicitaly test for
this case.

plugin/cache/cache.go

@@ -171,7 +171,7 @@ func (w *ResponseWriter) set(m *dns.Msg, key int, mt response.Type, duration tim
 	case response.OtherError:
 		// don't cache these
 	default:
-		log.Printf("[WARNING] Caching called with unknown classification: %d", mt)
+		log.Printf("[WARNING] Caching called with unknown typification: %d", mt)
 	}
 }