Cannot update `first_seen` or `last_seen` when its null

[righel]

When trying to update an attribute that has a null first_seen and last_seen and try to update them via PyMISP an error is returned:

Something went wrong (405): {'name': 'Attribute could not be saved: Attribute in the request not newer than the local copy.', 'message': 'Attribute could not be saved: Attribute in the request not newer than the local copy.', 'url': '/attributes/edit/<UUID>'}

If the first_seen or last_seen values are already set there is no error.

POC:

attribute = misp.get_attribute("<UUID>", pythonify=True)
attribute.first_seen = '2026-07-30T09:00:00.00000+07:00'
attribute.last_seen = '2026-07-30T10:00:00.00000+07:00'
# attribute.comment = 'Updated first_seen and last_seen timestamps'
misp.update_attribute(attribute, pythonify=True)

If we change something else in the attribute, such as the comment the attribute is properly updated.