Fix running schedule:test on CallbackEvent

[olivernybroe]

When running the schedule:test on a scheduler added with CallbackEvent

$schedule->call(MyScheduleClass::class . '@__invoke')->everyTenMinutes()

It shows up the following way in the schedule command.

However choosing one of the values results in the following

To fix I added a fallback to getSummaryForDisplay if no command property.

Which makes them show up the following way

And I can now actually run them 👍

A better solution could possibly be to add a new method which is responsible for generating the name for this, so it wouldn't need a fallback like this, however not sure if it is needed.