If you add a parallel branch of execution in the editor, add some steps, and save - it will save correctly, however it isn't a very good Jenkinsfile.
Loading it up once it has completed a run will result in an error to visualise. The Jenkinsfile will end up looking like:
Note that the branch name is empty string - this seems to upset declarative. I am not sure it executes that branch.
In any case, a branch should be required to be named, so I think this should be a validation error right? OR the editor should name that branch it creates.
To reproduce: take an existing stage with at least one step, add a parallel branch, add steps to it (but do not name it)