BUG/MEDIUM: http-ana: Don't eval http-after-response ruleset on empty messages

BUG/MEDIUM: http-ana: Don't eval http-after-response ruleset on empty messages

It is not possible on response comming from a server, but an errorfile may be
empty. In this case, the http-after-response ruleset must not be evaluated
because it is totally unexpected to manipulate headers on an empty HTX message.

This patch must be backported everywhere the http-after-response rules are
supported, i.e as far as 2.2.

(cherry picked from commit aab1b67383993b93bd70144825bc4350e3986a10)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>