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.
diff --git a/include/haproxy/errors.h b/include/haproxy/errors.h
index 4ece6e5..a3614b7 100644
--- a/include/haproxy/errors.h
+++ b/include/haproxy/errors.h
@@ -88,7 +88,7 @@
 #define HA_DIAG_WARNING_COND(cond, fmt, ...)                  \
 	do {                                                  \
 		if ((global.mode & MODE_DIAG) && (cond))      \
-			_ha_diag_warning((fmt), __VA_ARGS__); \
+			_ha_diag_warning((fmt), ##__VA_ARGS__); \
 	} while (0)
 
 /*