schedules: determine what to do with schedules with no trips

[tahini]

For example, when deleting paths from a line, it will delete the trips that were previously associated with this path. That may leave orphaned services, with no trips for this line. There will be a schedule in the schedule table and schedule periods in the periods table, but no trips.

What do we want to do with these schedules? Some options:

• Nothing: we leave it there with 0 trip and can edit it later, if/when we want to
• Add a warning upon saving the line: If we detect empty schedules, we can offer to keep them or delete them.
• Automatically delete when there are no trips.

imho the last is not a good option, as the periods may have been configured and the user may want to update the schedule in the same edit session. So we need to pick between the first 2 options.

[greenscientist]

Is it possible to access the schedule if there's no trip associated to it?