Unable to view consumer groups that have members with no assigned partitions

[rgrochowicz]

Hi all,

After upgrading our Helm chart from 0.7.10 to 3.3.0 (it's a big jump, yeah), we're getting this error when attempting to view a certain consumer group of ours:

Cannot read properties of null (reading 'length')
  https://redpanda.infra/static/js/async/2233.9aedb82d.js:42:31607
  https://redpanda.infra/static/js/async/2233.9aedb82d.js:234:443606
  Array.sum (https://redpanda.infra/static/js/async/2233.9aedb82d.js:234:443590)
  F.render (https://redpanda.infra/static/js/async/2233.9aedb82d.js:42:31585)
  ts (https://redpanda.infra/static/js/async/4925.1345c717.js:3:532235)
  https://redpanda.infra/static/js/async/4925.1345c717.js:3:515783
  tf (https://redpanda.infra/static/js/async/4925.1345c717.js:3:538406)
  e.track (https://redpanda.infra/static/js/async/4925.1345c717.js:3:543262)
  F.l [as render] (https://redpanda.infra/static/js/async/4925.1345c717.js:3:515752)

That tracks back to this line:

console/frontend/src/components/pages/consumers/group-details.tsx

Line 176 in 732ce68

const totalPartitions = group.members.flatMap((m) => m.assignments).sum((a) => a.partitionIds.length);

In the network response from /api/consumer-groups/app-prod, it's returning null for partitionIds:

{
    "consumerGroup": {
        "groupId": "app-prod",
        "state": "Stable",
        "protocolType": "consumer",
        "protocol": "faust",
        "members": [
            {
                "id": "faust-0.11.3-0bbc3830-827c-4a61-b631-16d10479eb34",
                "clientId": "faust-0.11.3",
                "clientHost": "/10.0.2.64",
                "assignments": [
                    {
                        "topicName": "app-prod-__assignor-__leader",
                        "partitionIds": null
                    }
                ]
            },
            // [...]
        ]
    ]
}

Environment
GIT_SHA         : dfd158892eb328e33af318eeec4c2b0ef25d9fb6
GIT_REF         : master
BUILD_TIMESTAMP : 1764254150
PLATFORM_VERSION: v25.3
BUILT_FROM_PUSH : false

Thanks!