[FIX] hr_holidays: handle tz attribute error if user timezone empty

When a user tries to create a leave request without linking an employee
with the user and without a configured login user timezone, a traceback
will be generated.

Steps to reproduce:
- Install the "hr_holidays" module.
- Create a new user, e.g., "Test user", and login with another browser.
- Login as an admin user and go to Settings > Users & Companies.
- Search for "Test user" and set Timzone as empty.
- Go to the Time Off menu and create a leave request; after that, a traceback
will be generated.
Error: AttributeError: 'bool' object has no attribute 'upper' 

When a user tries to create a leave request without linking an employee with the
user and without a configured login user timezone, the
_get_start_or_end_from_attendance() function of the "hr.leave" object will call
at that time timzone not getting.
Code reference:
https://github.com/odoo/odoo/blob/15.0/addons/hr_holidays/models/hr_leave.py#L111

Sentry-4441512696

closes odoo/odoo#134434

X-original-commit: 904edbdbbc5e11608d7e3f3f7d2d3001b6247ed2
Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>
Signed-off-by: Pradip Yenpure (prye) <prye@odoo.com>

addons/hr_holidays/models/hr_employee.py

@@ -308,7 +308,7 @@ class HrEmployee(models.Model):
     @api.model
     def get_public_holidays_data(self, date_start, date_end):
         self = self._get_contextual_employee()
-        employee_tz = pytz.timezone(self._get_tz() if self else self.env.user.tz)
+        employee_tz = pytz.timezone(self._get_tz() if self else self.env.user.tz or 'utc')
         public_holidays = self._get_public_holidays(date_start, date_end).sorted('date_from')
         return list(map(lambda bh: {
             'id': -bh.id,

addons/hr_holidays/models/hr_leave.py

@@ -1565,7 +1565,7 @@ class HolidaysRequest(models.Model):
 
     def _to_utc(self, date, hour, resource):
         hour = float_to_time(float(hour))
-        holiday_tz = timezone(resource.tz or self.env.user.tz)
+        holiday_tz = timezone(resource.tz or self.env.user.tz or 'UTC')
         return holiday_tz.localize(datetime.combine(date, hour)).astimezone(UTC).replace(tzinfo=None)
 
     def _get_attendances(self, request_date_from, request_date_to):