$ cat ./20.c
#include <ctype.h>
#include <stdio.h>
int main() {
            char ch; ch = '\20';
            printf("is byte \"%c\" NULL?\n", ch);
            if (ch == '\0') {
                printf("yes,               ch = \\%o\n", ch);                
            } else if (isascii(ch)) {
                printf("no (but is ascii), ch = '%c'\n", ch);
            } else {
                printf("no,                ch = \\%o\n", ch);
            }
}

$ gcc ./20.c -o 20


$ ./20
is byte "" NULL?
no (but is ascii), ch = ''