test_ssl_old: avoid empty strings for flags

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5396)

test/recipes/80-test_ssl_old.t

@@ -450,7 +450,8 @@ sub testssl {
                     note "*****SKIPPING $protocol $cipher";
                     ok(1);
                 } else {
-                    ok(run(test([@ssltest, @exkeys, "-cipher", $cipher, $flag])),
+                    ok(run(test([@ssltest, @exkeys, "-cipher", $cipher,
+                                 $flag || ()])),
                     "Testing $cipher");
                 }
             }