AssignmentHelper => Update validate for optional properties of immutables

helpers/Dojo/SharedAssignmentHelper.ts

@@ -15,9 +15,11 @@ class SharedAssignmentHelper {
 
                 immutable: {
                     elements: {
-                        properties: {
+                        properties        : {
+                            path: { type: 'string' }
+                        },
+                        optionalProperties: {
                             description: { type: 'string' },
-                            path       : { type: 'string' },
                             isDirectory: { type: 'boolean' }
                         }
                     }

types/Dojo/ImmutableFileDescriptor.ts

 interface ImmutableFileDescriptor {
-    description: string,
+    description?: string,
     path: string,
-    isDirectory: boolean,
+    isDirectory?: boolean,
 }