Guard against no namespace passed into a can/cannot helper

This can happen if a job doesn't have a namespace (which shouldn't
actually ever happen, but it can in tests).

ui/app/abilities/abstract.js

@@ -17,6 +17,7 @@ export default class Abstract extends Ability {
   namespace = 'default';
 
   get _namespace() {
+    if (!this.namespace) return 'default';
     if (typeof this.namespace === 'string') return this.namespace;
     return get(this.namespace, 'name');
   }