Summary: https://github.com/facebook/react-native/pull/8071 introduced a bug where jumping to the same navigator route would show a blank screen since the `index` check is not catching cases where the `transitionQueue` is empty.
Reviewed By: ericvicenti
Differential Revision: D3464697
fbshipit-source-id: 494527d1fb0ac5aea394abd3231dd19c56596549