Fix dags next-execution --table crash when no next schedule exists

[vincere-mori]

Summary

airflow dags next-execution <dag_id> --table crashes with AttributeError when the DAG has no next scheduled run (e.g. schedule=None, or an @once DAG after its run, or --num-executions exceeds available runs).

The root cause is in the table-output branch:

# Before (buggy)
AirflowConsole().print_as_table([{n: f(o) for n, f in getters} for o in iter_next_dagrun_info()])

iter_next_dagrun_info() yields DagRunInfo | None. When o is None, calling operator.attrgetter(c)(None) raises AttributeError: 'NoneType' object has no attribute 'logical_date'.

The non-table path already handles None correctly:

for info in iter_next_dagrun_info():
    if info is None:
        print("[WARN] No following schedule can be found. ...", file=sys.stderr)
        ...

Reproduce

# no_schedule_dag.py
from airflow import DAG
from pendulum import datetime

with DAG("no_schedule", start_date=datetime(2025, 1, 1, tz="UTC"), schedule=None):
    pass

airflow dags next-execution no_schedule --table
# AttributeError: 'NoneType' object has no attribute 'logical_date'

Fix

Mirror the non-table None guard in the table path: skip None items and print the same warning to stderr.

Fixes #67394

[boring-cyborg]

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