[FIX] calendar: date start

When creating and editing a new calendar.event record from the calendar
view, the start_datetime field does not prefill from the day that was
selected on the calendar.

Introduced in e08f999468

opw-1875276

addons/calendar/models/calendar.py

@@ -861,11 +861,13 @@ class Meeting(models.Model):
 
     @api.onchange('start_date')
     def _onchange_start_date(self):
-        self.start = self.start_date
+        if self.start_date:
+            self.start = self.start_date
 
     @api.onchange('stop_date')
     def _onchange_stop_date(self):
-        self.stop = self.stop_date
+        if self.stop_date:
+            self.stop = self.stop_date
 
     ####################################################
     # Calendar Business, Reccurency, ...