Always return an array from getCurrentUserGroups

Author: Tom Kirkpatrick

lib/utils.js

@@ -216,7 +216,7 @@ module.exports = class AccessUtils {
    */
   getCurrentUserGroups() {
     const ctx = this.app.loopback.getCurrentContext();
-    const currentUserGroups = ctx && ctx.get('currentUserGroups') || null;
+    const currentUserGroups = ctx && ctx.get('currentUserGroups') || [];
 
     if (ctx) {
       debug('currentUserGroups(): %o', currentUserGroups);