Handle no next run in dags next-execution --table

[fallintoplace]

What this does

• handle the None sentinel in airflow dags next-execution --table
• stop building table rows once there is no following schedule
• add regression tests for schedule=None and @once with --num-executions 2 --table

Why

The non-table path already handles the "no next run" case, but the table path iterated over DagRunInfo | None and applied attribute getters unconditionally. That could raise when a DAG has schedule=None, or when an @once DAG is asked for additional executions.

Closes #67394.

Testing

• Not run (not requested in this local workflow)

[boring-cyborg]

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