luaL_optlstring[-0, +0, v]
const char *luaL_optlstring (lua_State *L,
                             int arg,
                             const char *d,
                             size_t *l);
 If the function argument arg is a string, returns this string. If this argument is absent or is nil, returns d. Otherwise, raises an error. 
 If l is not NULL, fills the position *l with the result's length. If the result is NULL (only possible when returning d and d == NULL), its length is considered