C++ :: Point Of NULL In CString

I dont see any point of NULL in cstring. The code given below just outputs same as it would have done with NULL. My understanding is if size of char array is less than length of char array then null must be manually added?

#include <iostream>
using namespace std;
int main(){
char chr[0];
cin>>chr;//or if you use cin.getline;
cout<<chr<<endl;
return 0;
}

Enter something: hellowwwww
hellowwwww
Segmentation fault (core dumped)

why? for NULL char or something else?