allow a tc to be empty without the planner failing

When creating a hierarchical workflow, the root workflow may only contain SubWorkflow jobs, and therefore it isn't necessary to have a Transformation Catalog for the root workflow. This is the case with some LIGO workflows, and we should look into allowing an empty Transformation Catalog without having the planner fail.

notes: https://docs.google.com/document/d/1e6rfkJDiXN3eKhSma7V_PmihfsgcaK9hisfOtPK6KrY/edit