Plugin manager throws NPE when using update centers without categories

This is a regression after JENKINS-64196 . Although the majority of Jenkins users rely on the https://github.com/jenkins-infra/update-center2 instance to retrieve the update details, there are other update center implementations out there. They do not implement all new features of the Jenkins update center.

We do not have a strict update center API specification, and hence it is not possible to say what data would be returned by a particular update center. Some fields may be missing and end up as null values in the codebase. Categories is one of them, and hence https://github.com/jenkinsci/jenkins/pull/5051/files#diff-0dd3c7930da03922fff631a2cf06fbfda9b9bc688a4e5804fceda225147778a6R1399 may lead to NPE