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>
diff --git a/src/http_ana.c b/src/http_ana.c
index 5f81b08..deec220 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -4524,7 +4524,7 @@
 	if (final) {
 		htx->flags |= HTX_FL_PROXY_RESP;
 
-		if (!http_eval_after_res_rules(s))
+		if (!htx_is_empty(htx) && !http_eval_after_res_rules(s))
 			return 0;
 
 		if (s->txn->meth == HTTP_METH_HEAD)