rgw/sns: ListTopics uses account root arn for policy evaluation

rgw/sns: ListTopics uses account root arn for policy evaluation

when called by a non-root account user, permissions from identity policy
were not being applied correctly and always resulted in:
> evaluate_iam_policies: implicit deny from identity-based policy

passing a non-empty ARN argument to verify_user_permission() fixes this.
while other SNS APIs use a specific topic's arn, ListTopics doesn't
operate on individual topics so we use the account root user's arn

Fixes: https://tracker.ceph.com/issues/74595
Signed-off-by: Casey Bodley <cbodley@redhat.com>