Make path/listing optional (sort of).

Author: Peter Amstutz

draft-4/Process.yml

@@ -142,13 +142,13 @@ $graph:
         _type: "@vocab"
       doc: Must be `Directory` to indicate this object describes a Directory.
     - name: path
-      type: string
+      type: string?
       doc: The path to the directory.
       jsonldPredicate:
         _id: "cwl:path"
         _type: "@id"
     - name: listing
-      type: Dirent[]
+      type: Dirent[]?
       doc: List of files or subdirectories contained in this directory
       jsonldPredicate:
         _id: "cwl:listing"