MINOR: errors: allow empty va_args for diag variadic macro

Use the '##' operator to allow the usage of HA_DIAG_WARNING_COND macro
without extra arguments.

This must be backported up to 2.4.

(cherry picked from commit 01b3c3d4fb634ddf12ae103719b77f965987f037)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed