CLEANUP: http-htx: Set buffer area to NULL instead of malloc(0)

CLEANUP: http-htx: Set buffer area to NULL instead of malloc(0)

During error files conversion to HTX message, in http_str_to_htx(), if a
file is empty, the corresponding buffer's area is initialized with a
malloc(0) and its size is set to 0. There is no problem here. The behaviour
is totally defined. But it is not really intuitive. Instead, we can simply
set the area to NULL.

This patch should fix the issue #1022.