Don't show null URL previews

These are URLs that were spidered by the server without error but yielded an empty response from the server. There's nothing to display, so return an empty div.

src/components/views/rooms/LinkPreviewWidget.js

@@ -100,7 +100,9 @@ module.exports = React.createClass({
 
     render: function() {
         var p = this.state.preview;
-        if (!p) return <div/>;
+        if (!p || Object.keys(p).length === 0) {
+            return <div/>;
+        }
 
         // FIXME: do we want to factor out all image displaying between this and MImageBody - especially for lightboxing?
         var image = p["og:image"];