When using the build user vars plugin in a pipeline the current required usage is:
The 'node' is required here, because of how 'wrap' works. Because of this it is currently not possible to get the build user information outside the context of a node.
Creating a custom Workflow step would solve this. Example:
Or to keep it closer to how it currently works:
See https://issues.jenkins-ci.org/browse/JENKINS-30142 for a similar issue, which has been solved, for a different plugin.