Tests: unbreak stream_ssl_preread_alpn.t on OpenSSL < 1.0.2h.

Do not check return value for empty ALPN test case, since older OpenSSL
versions tolerate empty strings.  The test is still useful for coverage.

stream_ssl_preread_alpn.t

diff --git a/stream_ssl_preread_alpn.t b/stream_ssl_preread_alpn.t
index afeb310..751a8c2 100644
--- a/stream_ssl_preread_alpn.t
+++ b/stream_ssl_preread_alpn.t
@@ -100,7 +100,7 @@
 		or die "Can't create certificate for $name: $!\n";
 }
 
-$t->try_run('no ssl_preread_alpn_protocols')->plan(6);
+$t->try_run('no ssl_preread_alpn_protocols')->plan(5);
 
 ###############################################################################
 
@@ -114,9 +114,7 @@
 
 is(get_ssl(8081, 'foo', 'bar'), $p3, 'alpn many');
 
-# fallback to an empty value
-
-ok(!get_ssl(8081, ''), 'alpn empty');
+get_ssl(8081, '');
 
 ###############################################################################