[FIX] hr_work_entry_contract: compute duration without employee

This traceback arises when the user changes the 'end_date' without the employee
value, while creating a new 'Work Entry'.

To reproduce this issue:

1) Install 'hr_work_entry_contract'
2) Open 'payroll' and create a new 'Work Entry'
3) Select 'Work Entry Type' as 'Unpaid' and change the 'To' date

Error:- 'KeyError: False'

On the '_get_duration_batch' method, when the user is not given employee value,
there will be no 'contract_id' as it is getting value from 'employee_id'.

It leads to the above traceback as 'result[work_entry.id]' is getting value
through 'employee.id'.

See:- https://github.com/odoo/odoo/blob/a277faa2ffab7559fcbad95fcc1e8fd6a26d756b/addons/hr_work_entry_contract/models/hr_work_entry.py#L110-L112

Will resolve this issue by calculating duration without employee

sentry-4514970596

closes odoo#138033

X-original-commit: cc3898a
Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>

addons/hr_work_entry_contract/models/hr_work_entry.py

@@ -92,7 +92,7 @@ def _get_duration_batch(self):
         # {(date_start, date_stop): {calendar: employees}}
         mapped_periods = defaultdict(lambda: defaultdict(lambda: self.env['hr.employee']))
         for work_entry in self:
-            if not work_entry.date_start or not work_entry.date_stop or not work_entry._is_duration_computed_from_calendar():
+            if not work_entry.date_start or not work_entry.date_stop or not work_entry._is_duration_computed_from_calendar() or not work_entry.employee_id:
                 super_work_entries |= work_entry
                 continue
             date_start = work_entry.date_start