[FIX] hr_employee: synchronise if user is removed

Steps to reproduce:
-------------------
- create two employees (A and B);
- create a user;
- add the user in Related User of employee A;
- remove the user;
- add the user in Related User of employee B;
- change the Work Email of the employee B.

Issue:
------
The Work Email of the employee A is also updated.

Cause:
------
When we add a `user_id` to an employee,
we update the `work_contact_id` field.
Fields `mobile_phone` and `work_email` are inverse fields.
When we modify them, the `_inverse_work_contact_details`
method is called.
We update the `work_contact_id` linked to the employee.

When we delete an employee's `user_id`,
we don't update the `work_contact_id`.
Therefore, when we update an employee's `work_contact_id`,
we call the `_compute_work_contact_details` method
method for all employees who have the same `work_contact_id`.

The result is that we modify the `mobile_phone` and `ẁork_email`
fields for all employees linked to the `work_contact_id`.

Solution:
---------
Differentiate the case where the `user_id` is `False` (>< `None`)
when it is modified and does not contain
a value to "synchronise" the `work_contact_id`.

opw-3338188

closes odoo/odoo#123830

Signed-off-by: Kevin Baptiste <kba@odoo.com>

addons/hr/models/hr_employee.py

@@ -362,7 +362,7 @@ class HrEmployeePrivate(models.Model):
             self.message_unsubscribe(self.address_home_id.ids)
             if vals['address_home_id']:
                 self._message_subscribe([vals['address_home_id']])
-        if vals.get('user_id'):
+        if 'user_id' in vals:
             # Update the profile pictures with user, except if provided 
             vals.update(self._sync_user(self.env['res.users'].browse(vals['user_id']),
                                         (bool(self.image_1920))))

addons/hr/tests/test_hr_employee.py

@@ -181,3 +181,31 @@ class TestHrEmployee(TestHrCommon):
             'partner_id': partner.id,
         })
         self.assertFalse(self.env['res.users'].search([('login', '=', 'test_user')]).employee_id)
+
+    def test_employee_update_work_contact_id(self):
+        """
+            Check that the `work_contact_id` information is no longer
+            updated when an employee's `user_id` is removed.
+        """
+        user = self.env['res.users'].create({
+            'name': 'Test',
+            'login': 'test',
+            'email': 'test@example.com',
+        })
+        employee_A, employee_B = self.env['hr.employee'].create([
+            {
+                'name': 'Employee A',
+                'user_id': user.id,
+                'work_email': 'employee_A@example.com',
+            },
+            {
+                'name': 'Employee B',
+                'user_id': False,
+                'work_email': 'employee_B@example.com',
+            }
+        ])
+        employee_A.user_id = False
+        employee_B.user_id = user.id
+        employee_B.work_email = 'new_email@example.com'
+        self.assertEqual(employee_A.work_email, 'employee_A@example.com')
+        self.assertEqual(employee_B.work_email, 'new_email@example.com')