AssignmentHelper => Adapt to optional volume field

helpers/Dojo/SharedAssignmentHelper.ts

@@ -25,7 +25,9 @@ class SharedAssignmentHelper {
 
                 result: {
                     properties        : {
-                        container: { type: 'string' },
+                        container: { type: 'string' }
+                    },
+                    optionalProperties: {
                         volume: { type: 'string' }
                     }
                 }