Fix a crash in ViewPagerAndroid when passing a null child

Summary:
Fixes #5195
Closes https://github.com/facebook/react-native/pull/5236

Reviewed By: svcscm

Differential Revision: D2819197

Pulled By: androidtrunkagent

fb-gh-sync-id: cea451802c659512f64c1e90905647b8fbe4490b

Libraries/Components/ViewPager/ViewPagerAndroid.android.js

@@ -112,6 +112,9 @@ var ViewPagerAndroid = React.createClass({
     // will handle positioning of elements, so it's not important to offset
     // them correctly.
     return React.Children.map(this.props.children, function(child) {
+      if (!child) {
+        return null;
+      }
       var newProps = {
         ...child.props,
         style: [child.props.style, {