operator.identifier should never be nullable

[fzakfeld]

currently, operator.identifier can be null, for example

"operator": {
        "id": 3760,
        "identifier": null,
        "name": "S-Bahn Rhein-Main"
      },

This makes it hard to parse / compare it by a machine, e.g. for automations. As far as I understand, the id will be different for each instance. A (generated?) mandatory identifier like s-bahn-rhein-main would be good to have