Calendar view does not support empty date/time
The calendar view requires that the `dtstart` and `dtend` (if defined) have a value to be displayed. As we can not enforce that on the server side by definition (ex: the fields could `Function`), I think we should just filter out the records without values using the domain. For example, sao crashes when an event without `dtend` (so fullcalendar display it with a default size) with `TypeError: Cannot read properties of null (reading 'isDateTime')`.
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