MDL-28384 convert empty log urls to ''

The problem is that the log table does not allow nulls, credit goes to Troy Williams.

lib/datalib.php

@@ -1662,6 +1662,8 @@ function add_to_log($courseid, $module, $action, $url='', $info='', $cm=0, $user
     $info = $info;
     if (!empty($url)) { // could break doing html_entity_decode on an empty var.
         $url = html_entity_decode($url);
+    } else {
+        $url = '';
     }
 
     // Restrict length of log lines to the space actually available in the