MINOR: mux-quic: do not allocate Tx buf for empty STREAM frame

MINOR: mux-quic: do not allocate Tx buf for empty STREAM frame

Sometimes it may be necessary to send an empty STREAM frame to signal
clean stream closure with FIN bit set. Prior to this change, a Tx buffer
was allocated unconditionnally even if no data is transferred.

Most of the times, allocation was not performed due to an older buffer
reused. But if data were already acknowledge, a new buffer is allocated.
No memory leak occurs as the buffer is properly released when the empty
frame acknowledge is received. But this allocation is unnecessary and it
consumes a connexion Tx buffer for nothing.

Improve this by skipping buffer allocation if no data to transfer.
qcs_build_stream_frm() is now able to deal with a NULL out argument.

This should be backported up to 2.6.