Fix TicTacToe example layout

Summary:
I noticed that after starting a new game the board is not correctly layouted:

![-08-2017 21-39-21](https://cloud.githubusercontent.com/assets/3778452/21752532/06afc648-d5eb-11e6-9f8a-ccdad00d3a6f.gif)

Maybe this bug was introduced after the change of flex behaviour in 0.36.
Just replacing empty view with null fixed it.

**Test plan (required)**

 - launch the TicTacToe app
 - complete a round
 - press on new game

 a new board should be centred both vertically and horizontally
Closes https://github.com/facebook/react-native/pull/11775

Differential Revision: D4392024

Pulled By: ericvicenti

fbshipit-source-id: d2742d6ecafd786c38212adeb3c952f18a0e6f2b

Examples/TicTacToe/TicTacToeApp.js

@@ -158,7 +158,7 @@ class GameEndOverlay extends React.Component {
     var tie = board.tie();
     var winner = board.winner();
     if (!winner && !tie) {
-      return <View />;
+      return null;
     }
 
     var message;