Replace flagrant instances of .html('') with .empty().

Props OriginalEXE, SergeyBiryukov.
Fixes #27034.

Built from https://develop.svn.wordpress.org/trunk@31690


git-svn-id: http://core.svn.wordpress.org/trunk@31671 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/theme.js

@@ -900,7 +900,7 @@ themes.view.Themes = wp.Backbone.View.extend({
 	// and keeping theme count in sync
 	render: function() {
 		// Clear the DOM, please
-		this.$el.html( '' );
+		this.$el.empty();
 
 		// If the user doesn't have switch capabilities
 		// or there is only one theme in the collection