Fixed implicitly nullable params (#6616)

Author: huangdijia

src/Traits/MultiExec.php

@@ -23,7 +23,7 @@ trait MultiExec
      *
      * @return array|Redis
      */
-    public function pipeline(callable $callback = null)
+    public function pipeline(?callable $callback = null)
     {
         $pipeline = $this->__call('pipeline', []);
 
@@ -35,7 +35,7 @@ public function pipeline(callable $callback = null)
      *
      * @return array|Redis|RedisCluster
      */
-    public function transaction(callable $callback = null)
+    public function transaction(?callable $callback = null)
     {
         $transaction = $this->__call('multi', []);