BUG/MINOR: http_htx: Support empty errorfiles

BUG/MINOR: http_htx: Support empty errorfiles

Empty error files may be used to disable the sending of any message for specific
error codes. A common use-case is to use the file "/dev/null". This way the
default error message is overridden and no message is returned to the client. It
was supported in the legacy HTTP mode, but not in HTX. Because of a bug, such
messages triggered an error.

This patch must be backported to 2.0 and 1.9. However, the patch will have to be
adapted.