$ cat ./20.c
#include <ctype.h>
#include <stdio.h>
main() {
            char unsigned 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.c:3:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
 main() {
 ^~~~

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