UniqueConstrain isn't respected on ActionRoles

[zannkukai]

Package version (if known): 1.4.2

Describe the bug

It is possible to register multiple ActionRoles with same values despite the SQLAlchemy UniqueConstraint (https://github.com/inveniosoftware/invenio-access/blob/master/invenio_access/models.py#L115-L118) if argument is None.

Steps to Reproduce

access_actionsroles table initial state

from invenio_access import action_factory, ActionRoles
from invenio_accounts.models import Role
from invenio_db import db
import random

random_role = random.choice(Role.query.all())

action = action_factory('my-action')
action_arg = action_factory('my-action-arg', parameter=True)

for i in range(3):
    db.session.add(ActionRoles.allow(action, role=random_role))
    db.session.commit()

for i in range(3):
    try:     
        db.session.add(ActionRoles.allow(action_arg('argValue'), role=random_role))
        db.session.commit()
    except Exception as e:
        print('Error', str(e))  # You should see 2 SQLAlchemy errors

access_actionroles table after the script

Expected behavior

The UniqueConstraint key should be respected even if action doesn't have any argument.
Regarding this StackOverflow answer, it should be possible using 2 indexes (one if argument is null, second if argument is not null) : https://stackoverflow.com/a/57647018

Additional context

It should be the same for ActionUsers and ActionSystemRoles