[18.0][FIX] auditlog: Ensure display_name computation works with new record.

[amh-mw]

This was identified by running base module tests on an odoo:18 container with auditlog installed, yielding:

ERROR odoo odoo.addons.base.tests.test_display_name: FAIL: Subtest TestEveryModel.test_display_name_new_record [`_compute_display_name` doesn't work with new record (first onchange call).] (model='auditlog.http.session')
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/odoo/addons/base/tests/test_display_name.py", line 34, in test_display_name_new_record
    model.onchange({}, [], fields_spec)
  File "/usr/lib/python3/dist-packages/odoo/addons/web/models/models.py", line 1019, in onchange
    snapshot1 = RecordSnapshot(record, fields_spec)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/odoo/addons/web/models/models.py", line 1106, in __init__
    self.fetch(name)
  File "/usr/lib/python3/dist-packages/odoo/addons/web/models/models.py", line 1121, in fetch
    self[field_name] = self.record[field_name]
                       ~~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/odoo/models.py", line 7076, in __getitem__
    return self._fields[key].__get__(self)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1236, in __get__
    self.recompute(record)
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1463, in recompute
    apply_except_missing(self.compute_value, recs)
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1436, in apply_except_missing
    func(records)
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1485, in compute_value
    records._compute_field_value(self)
  File "/usr/lib/python3/dist-packages/odoo/models.py", line 5295, in _compute_field_value
    fields.determine(field.compute, self)
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 110, in determine
    return needle(*args)
           ^^^^^^^^^^^^^
  File "/usr/local/src/metricwise/oca/server-tools/auditlog/models/http_session.py", line 24, in _compute_display_name
    tz_create_date = fields.Datetime.context_timestamp(httpsession, create_date)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/odoo/fields.py", line 2400, in context_timestamp
    assert isinstance(timestamp, datetime), 'Datetime instance expected'
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Datetime instance expected

Note: The newly created tests/test_http.py failed to provoke the issue, so it is necessary to run base tests to confirm the fix.

[StefanRijnhart]

Thanks!