ubus: fix legacy empty reply format

The legacy ubus protocol must not include an empty object in the result
array if the invoked ubus procedure yielded no response.

This fixes compatibility with existing legacy ubus api clients that expect
this behaviour, LuCI's fs.js in particular.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

ubus.c

@@ -465,10 +465,14 @@ uh_ubus_request_cb(struct ubus_request *req, int ret)
 		uh_ubus_init_json_rpc_response(cl, &buf);
 		r = blobmsg_open_array(&buf, "result");
 		blobmsg_add_u32(&buf, "", ret);
-		c = blobmsg_open_table(&buf, NULL);
-		blob_for_each_attr(cur, du->buf.head, rem)
-			blobmsg_add_blob(&buf, cur);
-		blobmsg_close_table(&buf, c);
+
+		if (blob_len(du->buf.head)) {
+			c = blobmsg_open_table(&buf, NULL);
+			blob_for_each_attr(cur, du->buf.head, rem)
+				blobmsg_add_blob(&buf, cur);
+			blobmsg_close_table(&buf, c);
+		}
+
 		blobmsg_close_array(&buf, r);
 		uh_ubus_send_response(cl, &buf);
 		return;