[Fleet] Category field is empty for categories that only have prerelease packages

[jsoriano]

Kibana version: seen at least in main

Elasticsearch version: N/A

Server OS version: N/A

Browser version: N/A

Browser OS version: N/A

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

Describe the bug: Category label appears empty for packages whose category is only used by prerelease packages.

Steps to reproduce:

Seen with the elastic_connectors package in Kibana main branch (the package requires kibana 9).

1. Start kibana from main branch.
2. Go to the Integrations UI.
3. Enable search for prerelease packages.
4. Search for the "connectors" package.
5. Category is empty.

Expected behavior: Category shows the "Connector" category, as defined in the manifest of the package: https://github.com/elastic/integrations/blob/40459dce30cc7943edf8a5d4764801fa76878519/packages/elastic_connectors/manifest.yml#L10

Screenshots (if relevant):

(sorry for the quality, copied from other screenshot in a document)

Errors in browser console (if relevant): N/A

Provide logs and/or server output (if relevant): N/A

Any additional context:

"connectors" category is known by the spec, the registry and kibana.

The Integrations UI seems to be collecting these labels from https://epr.elastic.co/categories. If prerelease=true is not included, the response effectively doesn't include the "Connectors" category.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)