Return false if empty cat passed. Restores old behavior.

git-svn-id: https://develop.svn.wordpress.org/trunk@16478 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/category-template.php

@@ -221,6 +221,9 @@ function get_the_category_list( $separator = '', $parents='', $post_id = false )
  * @return bool True if the current post is in any of the given categories.
  */
 function in_category( $category, $post = null ) {
+	if ( empty( $category ) )
+		return false;
+
 	return has_term( $category, 'category', $post );
 }