Default the child element of Localized to null

If no child element is provided and <Localized /> tries to return
the children as a fallback, undefined is returned from the
component. This causes a React error, which is difficult to
track down. Instead, default the child element to null, which is
a valid return value for a React component.

Author: flut1

fluent-react/src/localized.js

@@ -79,7 +79,7 @@ export default class Localized extends Component {
 
   render() {
     const { l10n, parseMarkup } = this.context;
-    const { id, attrs, children: elem } = this.props;
+    const { id, attrs, children: elem = null } = this.props;
 
     // Validate that the child element isn't an array
     if (Array.isArray(elem)) {