Merge commit 'b78d1ab89c7e5a93101faf691b574907e4e50f19' into draft-4

Author: Peter Amstutz

cwltool/schemas/draft-2/CommonWorkflowLanguage.yml

@@ -982,7 +982,7 @@ $graph:
       specializeTo: "#CommandLineBinding"
   fields:
     - name: name
-      type: string
+      type: ["null", string]
       doc: "The type name being defined."