MINOR: mux-quic: Don't send an emtpy H3 DATA frame during zero-copy forwarding

It may only happens when there is no data to forward but a last stream frame
must be sent with the FIN bit. It is not invalid, but it is useless to send
an empty H3 DATA frame in that case.

src/mux_quic.c

@@ -3058,6 +3058,7 @@ static size_t qmux_strm_done_ff(struct stconn *sc)
 		goto end;
 	}
 
+	if (data)
 		data += sd->iobuf.offset;
 	total = qcs->qcc->app_ops->done_ff(qcs);