BUG/MEDIUM: mux-h1: Don't emit 0-CRLF chunk in h1_done_ff() when iobuf is empty

BUG/MEDIUM: mux-h1: Don't emit 0-CRLF chunk in h1_done_ff() when iobuf is empty

A chunk message transferred via zero-copy forwarding in H1 may be
corrupted. This only happens when the chunk size is not known during the
nego stage and when there is nothing to forward when h1_donn_ff() is
called. In this case, we always emit a chunk. Because there is nothing to
forward, a 0-CRLF is emitted in the middle of the message.

The issue occurred with the HTTP stats applet only.

A simple fix is to check the size of data in the iobuf before emitting a new
chunk in h1_done_ff(). However, we still try to send outgoing data because
when this happens, it is most of time because the H1 output buffer is almost
full.

This patch should fix the issue #2453. No backport needed.