In HTML5, the action attribute is no longer required. Remove this attribute when empty.

The admin HTML is served with the HTML5 doctype.

"The action and formaction content attributes, if specified, must have a value that is a valid non-empty URL potentially surrounded by spaces." 
http://www.w3.org/html/wg/drafts/html/master/forms.html#attr-fs-action

Props voldemortensen.
Fixes #30126.

Built from https://develop.svn.wordpress.org/trunk@31200


git-svn-id: http://core.svn.wordpress.org/trunk@31181 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/network/site-themes.php

@@ -167,7 +167,7 @@ if ( isset( $_GET['enabled'] ) ) {
 
 <p><?php _e( 'Network enabled themes are not shown on this screen.' ) ?></p>
 
-<form method="get" action="">
+<form method="get">
 <?php $wp_list_table->search_box( __( 'Search Installed Themes' ), 'theme' ); ?>
 <input type="hidden" name="id" value="<?php echo esc_attr( $id ) ?>" />
 </form>