Function opt_lstring 

pub fn opt_lstring(
    state: &mut LuaState,
    arg: i32,
    def: Option<&[u8]>,
) -> Result<Option<Vec<u8>>, LuaError>

Return the string at arg; if absent/nil return def.

C: LUALIB_API const char *luaL_optlstring(lua_State *L, int arg, const char *def, size_t *len)