Fix SQL escape method

modifié:         core/lib/Thelia/ImportExport/Export/Type/ProductPricesExport.php

core/lib/Thelia/ImportExport/Export/Type/ProductPricesExport.php

@@ -141,15 +141,25 @@ class ProductPricesExport extends ExportHandler
         return $data->loadModelCriteria($query);
     }
 
+    /**
+     * @param $str
+     * @return string
+     *
+     * Really escapes a string for SQL request.
+     */
     protected function real_escape($str)
     {
         $return = "CONCAT(";
         $len = strlen($str);
+
         for($i = 0; $i < $len; ++$i) {
             $return .= "CHAR(".ord($str[$i])."),";
         }
+
         if ($i > 0) {
             $return = substr($return, 0, -1);
+        } else {
+            $return = "\"\"";
         }
         $return .= ")";