Fix UIExplorer list

Summary:
public
On fresh install, UIExplorer produces a blank list due to the search text being null. This fixes that by replacing the null with an empty string.

Reviewed By: milend

Differential Revision: D2795411

fb-gh-sync-id: 4bdde5d4f9e88dd933c7946dcbfb8b591a54baf6

Examples/UIExplorer/UIExplorerListBase.js

@@ -40,7 +40,7 @@ class UIExplorerListBase extends React.Component {
         components: [],
         apis: [],
       }),
-      searchText: this.props.searchText,
+      searchText: this.props.searchText || '',
     };
   }