Currently, a traceback occurs when the compute method triggers with recordsets 
having `resources_ids` and no `start` date.

To reproduce this issue:

1) Install `appointment`
2) Open the `Gantt` view of any `appointment type`
3) Click on any column of the `Gantt` view to create a new `calendar event` 
4) Remove the `start` date

Error:
```
ValueError: Compute method failed to assign calendar.event(<NewId 0x7f585f8d91e0>,).on_leave_resource_ids
  File "odoo/http.py", line 2248, in __call__
    response = request._serve_db()
  File "odoo/http.py", line 1823, in _serve_db
    return self._transactioning(_serve_ir_http, readonly=ro)
  File "odoo/http.py", line 1843, in _transactioning
    return service_model.retrying(func, env=self.env)
  File "odoo/service/model.py", line 134, in retrying
    result = func()
  File "odoo/http.py", line 1821, in _serve_ir_http
    return self._serve_ir_http(rule, args)
  File "odoo/http.py", line 1828, in _serve_ir_http
    response = self.dispatcher.dispatch(rule.endpoint, args)
  File "odoo/http.py", line 2053, in dispatch
    result = self.request.registry['ir.http']._dispatch(endpoint)
  File "odoo/addons/base/models/ir_http.py", line 220, in _dispatch
    result = endpoint(**request.params)
  File "odoo/http.py", line 756, in route_wrapper
    result = endpoint(self, *args, **params_ok)
  File "addons/web/controllers/dataset.py", line 38, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "addons/web/controllers/dataset.py", line 34, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "odoo/api.py", line 458, in call_kw
    result = getattr(recs, name)(*args, **kwargs)
  File "addons/web/models/models.py", line 1011, in onchange
    todo = [
  File "addons/web/models/models.py", line 1014, in <listcomp>
    if field_name not in done and snapshot0.has_changed(field_name)
  File "addons/web/models/models.py", line 1128, in has_changed
    return self[field_name].keys() != set(self.record[field_name]._ids) or any(
  File "odoo/models.py", line 6610, in __getitem__
    return self._fields[key].__get__(self, self.env.registry[self._name])
  File "odoo/fields.py", line 2954, in __get__
    return super().__get__(records, owner)
  File "odoo/fields.py", line 1270, in __get__
    raise ValueError(f"Compute method failed to assign {missing_recs}.{self.name}")
```

When the user removes the `start` date it triggers a compute method `_compute_on_leave_resource_ids` 
in which both `self` and `resource_events` are equal when the recordset having `resource_id`.
Eliminating one from the other leads to an empty record.

Also, because of no `start` date, it fails to enter into the for loop to assign a value to the `on_leave_resource_ids`

https://github.com/odoo/enterprise/blob/e57706031390e4ee8ddd85873bef24c59b0e9404/appointment/models/calendar_event.py#L119-L130

Eventually, the compute method failed to assign the value for the `on_leave_resource_ids` field.

And `on_leave_resource_ids` is a `read-only` and `non-storable` field. 
Which leads to a traceback when `missing_recs_ids` have a value from below lines

https://github.com/odoo/odoo/blob/006e35a6efd0c5ae4c122008261899a7cc76f693/odoo/fields.py#L1266-L1270

Note:- A `TypeError` also occurs when we removing the end date

After applying this commit, it will resolve this issue by assigning a value to the field.

sentry-5469275451