added NULL to PDO insert

SE/se-lib/Core/Pdo.php

@@ -628,6 +628,7 @@ EOF_STRUCT_MYSQL;
 
 	public function convertValueToSqlSafe($value, $xsdType = null) {
 		if ('NOW()' === $value) return 'NOW()';
+		else if (NULL === $value) return 'NULL';
 		else return $this->quote($value, PDO::PARAM_STR);// TODO: use $sqlSchema if set
 	}