MINOR: mux-quic: prevent push frame for unidir streams

MINOR: mux-quic: prevent push frame for unidir streams

For the moment, unidirectional streams handling is not identical to
bidirectional ones in MUX/H3 layer, both in Rx and Tx path. As a safety,
skip over uni streams in qc_send.

In fact, this change has no impact because qcs.tx.buf is emptied before
we start using qcs_push_frame, which prevents the call to
qcs_push_frame. However, this condition will soon change to improve
bidir streams emission, so an explicit check on stream type must be
done.

It is planified to unify uni and bidir streams handling in a future
stage. When implemented, the check will be removed.