After investigating, it turned out that there were a couple of related errors:
The first is that if the dependency info can’t be parsed from the https://updates.jenkins.io/current/plugin-versions.json, it’s parsed from the manifest. I assumed that the "Plugin-Dependencies” field would always be present in the manifest, but apparently this is not the case. This caused a null pointer error and the “null” output string that was being seen earlier.
The second, related issue is that the reason the dependency information was unable to be parsed from the plugins-versions.json was because the requested plugin who dependency info failed was the "node-iterator-api” plugin (needed for the vsphere-cloud he requested) and the plugin-versions.json has dependency information for version 1.5.0, but the version of node-iterator-api that was requested was 1.5, which the tool current views as not equivalent. I had created a Jira task for a similar issue a while back: https://issues.jenkins-ci.org/browse/JENKINS-58537