fix for mysql export module (allow specifying a default without specifying a length)

lib/Doctrine/Export/Mysql.php

@@ -500,7 +500,7 @@ class Doctrine_Export_Mysql extends Doctrine_Export
     public function getDefaultFieldDeclaration($field)
     {
         $default = '';
-        if (isset($field['default']) && $field['length'] <= 255) {
+        if (isset($field['default']) && ( ! isset($field['length']) || $field['length'] <= 255)) {
             if ($field['default'] === '') {
                 $field['default'] = empty($field['notnull'])
                     ? null : $this->valid_default_values[$field['type']];