Revoke DAG specific permissions once access_control gets empty value

[kosteev]

This change is follow-up to this change #15311 and #15464.

Basically this change assures that DAG specific permissions are revoked once access_control field is emptied for DAG.

closes: #17508
related: #8609

[kosteev]

@jedcunningham As you were author of related changes, please, take a look.

[kosteev]

After a bit more thinking I actually realised the issue here.
The thing is that if we revoke permissions on access_control = None, that means it makes impossible to assign manually permissions on DAG resources to roles as they will be cleaned up after syncing DAG (if access_control is None).

[kosteev]

It might be good idea to implement this change hidden under some Airflow configuration property.
Closing pull-request as of now, for the future discussion and making decision about implementation.