If $group is empty in WP_Object_Cache::decr(), consider it to be 'default'.

This is consistent with the rest of the cache methods wrapped by
functions; the functions pass an empty string by default, hence
the need for this check.

props bananastalktome.
fixes #21327.



git-svn-id: https://develop.svn.wordpress.org/trunk@21294 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/cache.php

@@ -323,6 +323,9 @@ class WP_Object_Cache {
 	 * @return false|int False on failure, the item's new value on success.
 	 */
 	function decr( $key, $offset = 1, $group = 'default' ) {
+		if ( empty( $group ) )
+			$group = 'default';
+		
 		if ( ! $this->_exists( $key, $group ) )
 			return false;