event: Set dt_end to next day when DTEND is not defined

from RFC5545 (p. 54) https://datatracker.ietf.org/doc/html/rfc5545#page-54
For cases where a "VEVENT" calendar component specifies a "DTSTART"
property with a DATE value type but no "DTEND" nor "DURATION" property,
the event's duration is taken to be one day.

Fix https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/327

src/core/gcal-event.c

@@ -368,6 +368,7 @@ setup_component (GcalEvent  *self,
   if (!end || !e_cal_component_datetime_get_value (end))
     {
       self->all_day = TRUE;
+      self->dt_end = g_date_time_add_days (self->dt_start, 1);
     }
   else
     {