There is no standardized NUL macro, at least not in C.
Also, NULL has the advantage over a plain 0 literal that it signals to the reader that "hey, we're dealing with a pointer" which might be considered an advantage.
In C++, the idiom was for a long while to just write 0, but then they seem to have reversed that and introduced a new literal, nullptr for some reason(s).
In C, I recommend writing NULL for pointers and '\0' for characters.
