[Bug]: Return updated operations on empty checkpoint

[yaythomas]

Expected Behavior

When creating a Checkpoint with an empty updates array, such that

 CheckpointDurableExecutionRequest(DurableExecutionArn=<<my-arn>>, CheckpointToken=<<my-token>>, Updates=[])

The response should contain NewExecutionState, where

If NO new events occurred:

1. CheckpointToken: New token with incremented sequence
2. NewExecutionState.Operations: Empty array []
3. NewExecutionState.NextMarker: null

If new events occurred (timer fired, etc.):

1. CheckpointToken: New token with incremented sequence
2. NewExecutionState.Operations: Array with those operations
3. NewExecutionState.NextMarker: null

The response CAN return an empty NewExecutionState.Operations array when there are no updates AND no new backend events.

The response should ALWAYS includes NewExecutionState - it never omits it.

Actual Behavior

The local-runner has a short-circuit only to pass back the updated checkpoint token and set new_execution_state=None if updates are empty. This is wrong.

        if updates:
            checkpoint_output = self._checkpoint_processor.process_checkpoint(
                checkpoint_token=checkpoint_token,
                updates=updates,
                client_token=client_token,
            )

            new_execution_state = None
            if checkpoint_output.new_execution_state:
                new_execution_state = CheckpointUpdatedExecutionState(
                    operations=checkpoint_output.new_execution_state.operations,
                    next_marker=checkpoint_output.new_execution_state.next_marker,
                )

            return CheckpointDurableExecutionResponse(
                checkpoint_token=checkpoint_output.checkpoint_token,
                new_execution_state=new_execution_state,
            )

        # Save execution state after generating new token
        new_checkpoint_token = execution.get_new_checkpoint_token()
        self._store.update(execution)

        return CheckpointDurableExecutionResponse(
            checkpoint_token=new_checkpoint_token,
            new_execution_state=None,
        )

Steps to Reproduce

Create empty checkpoint call

SDK Version

1

Python Version

3.13

Is this a regression?

No

Last Working Version

No response

Additional Context

No response