Merge branch 'michal/silence-a-perl-warning-output-by-stop.pl' into 'master'

Silence a Perl warning output by stop.pl

See merge request isc-projects/bind9!1649

bin/tests/system/stop.pl

@@ -240,7 +240,7 @@ sub clean_pid_file {
 	if (send_signal(0, $pid) == 0) {
 		# XXX: on windows this is likely to result in a
 		# false positive, so don't bother reporting the error.
-		if ($ENV{'CYGWIN'} eq "") {
+		if (!defined($ENV{'CYGWIN'})) {
 			print "I:$test:$server crashed on shutdown\n";
 			$errors = 1;
 		}