BUG/MINOR: http-ana: Do a L7 retry on read error if there is no response

BUG/MINOR: http-ana: Do a L7 retry on read error if there is no response

A regression about "empty-response" L7 retry was introduced with the commit
dd6496f591 ("CLEANUP: http-ana: Remove useless if statement about L7
retries").

The if statetement was removed on a wrong assumption. Indeed, L7 retries on
status is now handled in the HTTP analysers. Thus, the stream-connector
(formely the conn-stream, and before again the stream-interface) no longer
report a read error to force a retry. But it is still possible to get a read
error with no response. In this case, we must perform a retry is
"empty-response" is enabled.

So the if statement is re-introduced, reverting the cleanup.

This patch should fix the issue #2061. It must be backported as far as 2.4.