Related to https://github.com/grafana/oncall-private/issues/2826
RBAC enabled or not (OSS or cloud), it is possible to get service
account permissions, enabling perm check (for service account tokens) in
public API.
Also allow empty value for users in sync (instead of returning a 400
response).
There is no user to be set when creating a webhook via the public API if
authenticated using service account tokens (insight logs will still keep
track of the service account information behind the request).
