fix: panic when streaming release if using msgstream (#39374)

issue: #39367

Signed-off-by: chyezh <chyezh@outlook.com>

internal/distributed/streaming/wal.go

@@ -163,7 +163,9 @@ func (w *walAccesserImpl) Close() {
 	}
 	w.producerMutex.Unlock()
 
-	w.handlerClient.Close()
+	if w.handlerClient != nil {
+		w.handlerClient.Close()
+	}
 	w.streamingCoordClient.Close()
 }