SLAMiss is nullable and not always given back when pulling task instances

[csm10495]

related: apache/airflow-client-python#50

It appears as though the API doesn't have to return an SLAMiss value, so therefore it can be nullable. Without this change (and rebuilding of the client) we get a failure like so:

>>> ti = TaskInstanceApi(api_client)
>>> ti.get_task_instances('tutorial', 'manual__2022-10-31T20:32:58.139267+00:00')
...
ApiValueError: Invalid inputs given to generate an instance of 'TaskInstanceCollectionAllOf'. The input data was invalid for the allOf schema 'TaskInstanceCollectionAllOf' in the composed schema 'TaskInstanceCollection'. Error=Invalid type for variable 'sla_miss'. Required value type is SLAMiss and passed type was NoneType at ['received_data']['task_instances'][0]['sla_miss']

With this change (and rebuilding of the client) it works fine.

In [6]: ti.get_task_instances('tutorial', 'manual__2022-10-31T20:32:58.139267+00:00')
Out[6]:
{'task_instances': [{'dag_id': 'tutorial',
                     'dag_run_id': 'manual__2022-10-31T20:32:58.139267+00:00',
                     'duration': 0.116132,
                     'end_date': '2022-10-31T20:32:59.352991+00:00',
                     'execution_date': '2022-10-31T20:32:58.139267+00:00',
                     'executor_config': '{}',
...