[navPush] Check if the NavPush page exists.

When using the NavPush directive there isn't a way to disable the nav from trying to push.

src/components/nav/nav-push.ts

@@ -71,7 +71,7 @@ export class NavPush {
  */
   @HostListener('click')
   onClick(): boolean {
-    if (this._nav) {
+    if (this._nav && this.navPush) {
       this._nav.push(this.navPush, this.navParams, null);
       return false;
     }