[FIX] hr_holidays: avoid execution of onchange when no employee linked

If the current user is not linked to any employee we cannot retrieve his
calendar and so we cannot compute default times for the leave.

addons/hr_holidays/models/hr_leave.py

@@ -146,6 +146,9 @@ class HolidaysRequest(models.Model):
         date_from = False
         date_to = False
 
+        if not self.employee_id:
+            return
+
         if self.request_date_from:
             if self.date_from:
                 date_from = fields.Datetime.to_string(datetime.combine(fields.Date.from_string(self.request_date_from), fields.Datetime.from_string(self.date_from).time()))