Add "null" to to_string method

SquiLu/squirrel/sqvm.cpp

@@ -368,6 +368,9 @@ bool SQVM::ToString(const SQObjectPtr &o,SQObjectPtr &res)
     case OT_BOOL:
         scsprintf(_sp(sq_rsl(6)),sq_rsl(6),_integer(o)?_SC("true"):_SC("false"));
 		break;
+    case OT_NULL:
+        scsprintf(_sp(sq_rsl(5)),sq_rsl(5),_SC("null"));
+        break;
 	case OT_TABLE:
 	case OT_USERDATA:
 	case OT_INSTANCE: