BUG/MAJOR: mux-h1: Don't pretend the input channel's buffer is full if empty

BUG/MAJOR: mux-h1: Don't pretend the input channel's buffer is full if empty

A regression was introduced by the commit 76014fd1 ("MEDIUM: h1-htx: Add HTX EOM
block when the message is in H1_MSG_DONE state"). When nothing is copied in the
channel's buffer when the input message is parsed, we erroneously pretend it is
because there is not enough room by setting the CS_FL_WANT_ROOM flag on the
conn-stream. This happens when a partial request is parsed. Because of this
flag, we never try anymore to get input data from the mux because we first wait
for more room in the channel's buffer, which is empty. Because of this bug, it
is pretty easy to freeze a h1 connection.

To fix the bug, we must obsiously set the CS_FL_WANT_ROOM flag only when there
are still data to transfer while the channel's buffer is not empty.

This patch must be backported if the patch 76014fd1 is backported too. So for
now, no backport needed.