[FIX] calendar: edition of events duration

In event not during all day, it looks like
`start_date` could be defined, but `stop_date` not.

Despite the fact this is not such a normal state, this should
not prevent the save of the event with the
`_check_closing_date` constraint, checking that the ending
date is after the starting date
(in the case of an event not during the whole day,
this is ensured with the `start_datetime` and the
`stop_datetime`

opw-676245