Fix ContinuousTimetable false triggering when last run ends in future

[jx2lee]

closes: #45081

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[uranusjr]

if last_automated_data_interval.end > current_time:
    return None
start = ...
end = ...

Exactly the same logic, but more readable IMO.

[uranusjr]

Is returning None the right thing to do here? It would make this DAG not run at all in the future, even after the previous run is no longer in the future.

[jx2lee]

I missed that return None prevents the DAG from running anymore. So, how about applying start and end as shown below when the start_date is in the future(instead of None)?

        if last_automated_data_interval is not None:  # has already run once
            if last_automated_data_interval.end > current_time:  # start date is future
                start = restriction.earliest
                elapsed = last_automated_data_interval.end - last_automated_data_interval.start # elapsed already run

                end = start + elapsed.as_timedelta() # set end to "start + timedelta", not current_time
            else:
                start = last_automated_data_interval.end
                end = current_time

start is set to restriction.earliest, and end is calculated by adding the previous execution's time difference to start. This way, even if it's set in the future, expect the DAG to run. Could this approach cause any problems? (Also, since date calculations are involved, would it be better to put it in a separate function?)