[Upgrade Assistant] does not work if spaces are disabled

[breml]

Kibana version: 7.17.8

Elasticsearch version: 7.17.8

Server OS version: Alma Linux 8

Browser version: Chrome 109.0.5414.120

Browser OS version: Windows 10

Original install method (e.g. download page, yum, from source, etc.): yum

Describe the bug:

In Kibana the Upgrade Assistant page stays blank.

Steps to reproduce:

1. Add xpack.spaces.enabled: false to the Kibana config.
2. Restart Kibana
3. Access the Upgrade Assistant in Kibana.

Expected behavior:

The Upgrade Assistant should work regardless of the fact, if spaces are enabled or disabled or Kibana should return a meaningful error indicating, that spaces are required for the Upgrade Assistant to work (or this behavior needs to be clearly documented).

Errors in browser console (if relevant): No error is shown, also in the network tab of the development colsole no error is shown. The request to /api/upgrade_assistant/cluster_upgrade_status returns 200 OK status code with an empty response body.

Any additional context:

Maybe related:

• [Meta] Make spaces plugin optional #149687
• [Upgrade Assistant] infra should be optional dependency #111860

[elasticmachine]

Pinging @elastic/platform-deployment-management (Team:Deployment Management)

[alisonelizabeth]

Needs triage. I'm not sure that this is relevant anymore.

[SoniaSanzV]

I don't think it longer applies. From version 8.0.0 users can no longer set spaces.enabled to false: #82467