Machines can't be deployed after deselecting all archs in the "Ubuntu extra architectures" package repo

Deselecting all archs in the "Ubuntu extra architectures"/ubuntu-ports package repo blocks new deployments from happening with the following error:

"Error:Failed to retrieve curtin config: 'NoneType' object has no attribute 'url'"

When deselecting all archs and saving, the default archs are pulled in again (["amd64", "i386"]), which are filtered out by PackageRepository.get_ports_archive() because it looks for PORTS_ARCHES (["armhf", "arm64", "ppc64el", "s390x"]), triggering the error above.

The message doesn't give any information about the origin of the error. Given that this is a default repo, changing the archs pulled in when left empty or raising a warning when saving the changes seem to be good options for this bug.

Traceback from logs:

2020-11-25 16:29:01 maasserver.websockets.protocol: [critical] Error on request (49) machine.action: Failed to retrieve curtin config: 'NoneType' object has no attribute 'url'
        Traceback (most recent call last):
          File "/usr/lib/python3.8/threading.py", line 870, in run
            self._target(*self._args, **self._kwargs)
          File "/snap/maas/10643/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 825, in worker
            return target()
          File "/snap/maas/10643/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
            task()
          File "/snap/maas/10643/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
            task()
        --- <exception caught here> ---
          File "/snap/maas/10643/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/snap/maas/10643/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/snap/maas/10643/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/snap/maas/10643/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/snap/maas/10643/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 860, in callInContext
            return func(*args, **kwargs)
          File "/snap/maas/10643/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 192, in wrapper
            result = func(*args, **kwargs)
          File "/snap/maas/10643/lib/python3.8/site-packages/maasserver/utils/orm.py", line 737, in call_within_transaction
            return func_outside_txn(*args, **kwargs)
          File "/snap/maas/10643/lib/python3.8/site-packages/maasserver/utils/orm.py", line 540, in retrier
            return func(*args, **kwargs)
          File "/usr/lib/python3.8/contextlib.py", line 75, in inner
            return func(*args, **kwds)
          File "/snap/maas/10643/lib/python3.8/site-packages/maasserver/websockets/base.py", line 402, in prep_user_execute
            return self._call_method_track_queries(
          File "/snap/maas/10643/lib/python3.8/site-packages/maasserver/websockets/base.py", line 426, in _call_method_track_queries
            result = method(params)
          File "/snap/maas/10643/lib/python3.8/site-packages/maasserver/websockets/handlers/machine.py", line 945, in action
            return action.execute(**extra_params)
          File "/snap/maas/10643/lib/python3.8/site-packages/maasserver/node_action.py", line 171, in execute
            self._execute(*args, **kwargs)
          File "/snap/maas/10643/lib/python3.8/site-packages/maasserver/node_action.py", line 540, in _execute
            raise NodeActionError("Failed to retrieve curtin config: %s" % e)
        maasserver.exceptions.NodeActionError: Failed to retrieve curtin config: 'NoneType' object has no attribute 'url'