Bugfix: Allow getting details of a DAG with null start_date(REST API)

[ephraimbuddy]

Currently, when you request for dag details of a DAG with no start_date, it throughs an error:

{
    "detail": "None is not of type 'string'\n\nFailed validating 'type' in schema['allOf'][1]['properties']['start_date']:\n    {'format': 'date-time', 'readOnly': True, 'type': 'string'}\n\nOn instance['start_date']:\n    None",
    "status": 500,
    "title": "Response body does not conform to specification",
    "type": "http://apache-airflow-docs.s3-website.eu-central-1.amazonaws.com/docs/apache-airflow/latest/stable-rest-api-ref.html#section/Errors/Unknown"
}

This PR resolves this by nullifying the start_date of DAG detail schema in openapi

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[ephraimbuddy]

Please @mik-laj, can you take a look at this PR. I have a situation where the Get dag details endpoint fails when a DAG is triggered with the execution date in the future or when the task start date is before the dag start date

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest master or amend the last commit of the PR, and push it with --force-with-lease.

[kaxil]

Thanks @ephraimbuddy