MINOR: quic: skip sending if no frame to send in io-cb

MINOR: quic: skip sending if no frame to send in io-cb

Check on quic_conn_io_cb() if sending is required. This allows to skip
over Tx buffer allocation if not needed.

To implement this, we check if frame lists on current and next
encryption level are empty. We also need to check if there is no need to
send ACK, PROBE or CONNECTION_CLOSE. This has been isolated in a new
function qc_need_sending() which may be reuse in some other functions in
the future.