use warnings;
use Inline C => Config => BUILD_NOISY => 1;

use Inline C => <<'EOC';

void foo1(SV *x) {
     printf("%s\n", SvPV_nolen(x));
}

EOC

$x = undef;
foo1($x);
foo2($x);

sub foo2 {
    if(defined($x)) {foo1($x)}
    else {foo1("\0")}
}