bugfix: NavigatoriOS doesn't update any scenes when 0th scene is repl…

…aced

Summary:
Calling navigator.replace(0, scene) has no effect.

This is because 0 is false in Javascript so when

    this.state.updatingAllIndicesAtOrBeyond == 0

(meaning update all indices starting with 0)

The whole expression evaluates to 0, i.e. false ->  therefore no update
happens.

Explicitly checking for not-equal to null (!= will convert undefined to null automatically) fixes the issue.
Closes #5155

Reviewed By: svcscm

Differential Revision: D2807397

Pulled By: nicklockwood

fb-gh-sync-id: 519a4ab35c86b0b608808b36593f5f8c2ecd1561

Libraries/Components/Navigation/NavigatorIOS.ios.js

@@ -639,7 +639,7 @@ var NavigatorIOS = React.createClass({
     var {component, wrapperStyle, passProps, ...route} = route;
     var {itemWrapperStyle, ...props} = this.props;
     var shouldUpdateChild =
-      this.state.updatingAllIndicesAtOrBeyond &&
+      this.state.updatingAllIndicesAtOrBeyond != null &&
       this.state.updatingAllIndicesAtOrBeyond >= i;
     var Component = component;
     return (