FetchAndLock externalTasks via REST without topics

Hello,
according to the REST API-docs it should be possible to fetchAndLock external tasks without giving specific topics.

As soon as I remove my list of topics, I only get empty results. it doesn’t matter if I provide null as requested or remove the key entirely, always empty results.
My expectation was - if I read the API correctly - that I would receive ALL external tasks.
Tried it on 7.20 and 7.21.

Where am I going wrong?
Thanks and have a nice day.

This works as expected:

{
    "workerId": "f61904d0-4580-4365-aa59-fab3129b9128",
    "maxTasks": 10,
    "usePriority": true,
    "topics": [
        {
        "topicName": "myTopic",
        "lockDuration": 10000
        }
    ]
}

Empty result:

{
    "workerId": "f61904d0-4580-4365-aa59-fab3129b9128",
    "maxTasks": 10,
    "usePriority": true,
    "topics": null
}

Empty result:

{
    "workerId": "f61904d0-4580-4365-aa59-fab3129b9128",
    "maxTasks": 10,
    "usePriority": true
}