It is possible that a scheduled channel ended up with 0 bytes in its
outbuf after the scheduling loop and having an outbuf table entry
indicating that we need to flush bytes on the wire after the loop.
This lead to attempt to write 0 bytes up to the TLS layer that would
prevent such action.
All in all, this fixes wasted CPU cycles on attempting to flush nothing.
Fixes#40548
Signed-off-by: David Goulet <dgoulet@torproject.org>
