Avoid blank output in tls-nextprotoneg when server doesn't offer any protocols

scripts/tls-nextprotoneg.nse

@@ -116,7 +116,12 @@ local check_npn = function(response)
       table.insert(results, protocol)
     end
 
-    return results
+    if next(results) then
+      return results
+    else
+      stdnse.debug1("Server supports TLS NPN extension, but no protocols were offered.")
+      return "<empty>"
+    end
   else
     stdnse.debug1("Server response was not server_hello")
     return nil