fix(core): check key on storage event argument exists

When localStorage of application is cleared manually on the fly, this will break since event.key === null. Hence added in a check if it exists as well.

modules/core/src/storageinterruptsource.ts

@@ -14,7 +14,7 @@ export class StorageInterruptSource extends WindowInterruptSource {
    * @return True if the event should be filtered (don't cause an interrupt); otherwise, false.
    */
   filterEvent(event: StorageEvent): boolean {
-    if (event.key.indexOf('ng2Idle.') >= 0 && event.key.indexOf('.expiry') >= 0) {
+    if (event.key && event.key.indexOf('ng2Idle.') >= 0 && event.key.indexOf('.expiry') >= 0) {
       return false;
     }
     return true;