[MINOR] http: improve url_param pattern extraction to ignore empty values

It's better to avoid sticking on empty parameter values, as this almost
always indicates a missing parameter. Otherwise it's easy to enter a
situation where all new visitors stick to the same server.

src/proto_http.c

@@ -8340,7 +8340,7 @@ find_url_param_value(char* path, size_t path_l,
 
 	*value = value_start;
 	*value_l = value_end - value_start;
-	return 1;
+	return value_end != value_start;
 }
 
 static int