NPE in FolderWorkflow#add() when a query contains modify-parameters that are not supplied in the call

A query can have hippostd:modify properties. The value of fields in a document can be filled by workflow using these. When there is a property mentioned that is not used in the call to the workflow, a NPE occurs.

Steps to reproduce the error:
1. In the console of GoGreen go to /hippo:configuration/hippo:queries/hippo:templates/new-faq.
2. Add 2 new items to the hippostd:modify property: "./hippogogreen:question" and "$myvariable". Write changes to repository.
3. Log into the cms, and add a new faq in the FAQs folder. The message is: " An error occured, please retry". The stacktrace points to a NPE in org.hippoecm.repository.standardworkflow.FolderWorkflowImpl.

I think this class tries to parse the given argument, but no value was given during the call. I would expect that in that case the argument would be ignored. In any case the code should not break with an NPE.