BUG/MEDIUM: hlua: Don't set EOM flag on an empty HTX message in HTTP applet

BUG/MEDIUM: hlua: Don't set EOM flag on an empty HTX message in HTTP applet

In a lua HTTP applet, when the script is finished, we must be sure to not
set the EOM on an empty message. Otherwise, because there is no data to
send, the mux on the client side may miss the end of the message and
consider any shutdown as an abort.

See "UG/MEDIUM: stats: Be sure to never set EOM flag on an empty HTX
message" for details.

This patch must be backported as far as 2.4. On previous version there is
still the EOM HTX block.