[fix] alpn_protocol might return nil

Author: kares

src/main/java/org/jruby/ext/openssl/SSLSocket.java

@@ -242,8 +242,9 @@ private SSLEngine ossl_ssl_setup(final ThreadContext context, final boolean serv
     public final SSLContext context() { return this.sslContext; }
 
     @JRubyMethod(name = "alpn_protocol")
-    public final RubyString alpn_protocol(final ThreadContext context) {
-        return RubyString.newString(context.runtime, this.engine.getApplicationProtocol());
+    public IRubyObject alpn_protocol(final ThreadContext context) {
+        final String protocol = engine.getApplicationProtocol();
+        return protocol == null ? context.nil : RubyString.newString(context.runtime, protocol);
     }
 
     @JRubyMethod(name = "sync")