Fixes #3829 - Avoid sending empty trailer frames for http/2 responses.

Added guard against the supplier of trailers returning null.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

jetty-http2/http2-server/src/main/java/org/eclipse/jetty/http2/server/HttpTransportOverHTTP2.java

@@ -215,6 +215,8 @@ public class HttpTransportOverHTTP2 implements HttpTransport
         if (supplier == null)
             return null;
         HttpFields trailers = supplier.get();
+        if (trailers == null)
+            return null;
         return trailers.size() == 0 ? null : trailers;
     }