Correct type of $batchId in Batchable trait (#58069)

The `$batchId` type is incorrect. If no Batch is used the `$batchId` can be `null`.

Author: markieo1

src/Illuminate/Bus/Batchable.php

@@ -12,7 +12,7 @@ trait Batchable
     /**
      * The batch ID (if applicable).
      *
-     * @var string
+     * @var string|null
      */
     public $batchId;