BUG/MAJOR: mux-quic: properly fix BUG_ON on empty STREAM emission

BUG/MAJOR: mux-quic: properly fix BUG_ON on empty STREAM emission

Properly fix BUG_ON() occurence when QUIC MUX emits only empty STREAM
frames. This was addressed by a previous patch but it causes another
regression so a revert was needed.

BUG_ON() on qcc_build_frms() return value is invalid. Indeed,
qcc_build_frms() may return 0, but this does not imply that frame list
is empty, as encoded frames can have a zero length payload. As such,
simply remove this invalid BUG_ON().

This must be backported up to 3.1.

(cherry picked from commit ddfd8031f895b448604cbc259f74e6cb7b086b0b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>