[13.x] Fix batch finally callbacks after then deletes batch

[Button99]

This PR fixes a TypeError that can occur when a batch then() callback deletes the batch and a finally() callback is also registered.

Batch::invokeCallbacks() previously assumed $this->fresh() always returned a Batch instance. If the then() callback deleted the persisted batch row, the later finally() callback received null, violating invokeHandlerCallback()'s non-nullable Batch parameter.

This now falls back to the current in-memory batch instance when fresh() returns null.

[taylorotwell]

Honestly the code should probably just bail if the batch record has been deleted?