Throw an error instead of crashing app when video source is empty

If an undefined source is accidentally loaded in react-native-video a fatal error SIGABRT will happen and crash the application. This is very difficult to debug since there's apparently no relation between this crash and react-native-video. So this change checks if the URI is empty and, if it is, it throws an error, allowing the user to find out early about the issue.

https://cloud.githubusercontent.com/assets/5795227/20283860/08223ba6-aabb-11e6-9fdc-d12d32b3aa9a.png