Fix macro expansion of empty args.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

tests/tests2/71_macro_empty_arg.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+
+#define T(a,b,c) a b c
+
+int main(void)
+{
+    printf("%d", T(1,+,2) T(+,,) T(,2,*) T(,7,) T(,,));
+    return 0;
+}