[luabind] Undifined String Parameter NULL or ""

[jiang.yu\(mos\) <mmo...@16...>]

Hi!
I have a function 	c++ side regist by luabind like:

void Hi(const char* szHi)
{
	cout << szHi << endl;
}

and at lua side, when call Hi(temp); // temp not define.
will call Hi(NULL) and then the program will crash.

I wonder if I can modify some code of luabind when call Hi(temp); // temp
not define.
will call Hi(""), and then the program just donothing.

Best Regards
mos