BUG/MEDIUM: h3: do not block FIN on empty DATA frame

BUG/MEDIUM: h3: do not block FIN on empty DATA frame

A HTTP/3 DATA frame may be empty as its payload is entirely optional.
However, prior to this patch, H3 stream parsing was interrupted in this
case, unless other frames follow. The main problem is that it would lead
to a missing EOM if QUIC FIN is reported with an empty DATA frame,
causing the stream to freeze until timeout.

The first step of this patch is to first remove the early return for
empty HTTP/3 frame payload. This has the effect to allow calling the
different frame handlers with an empty payload. The current patch is
dedicated to HTTP/3 DATA support; other patches will follow to ensure
the safety of the other frame types handlers.

The second part of this patch is to adjust h3_data_to_htx() which is the
HTTP/3 data frame handler. It is now safe to call it with an empty
frame, with or without FIN. It will default to a noop operation, unless
FIN is set, in which case htx_set_eom() is used to report the closure of
the stream read channel.

This must be backported up to 2.6.