Allow empty macro arguments

test/macro.c

@@ -213,6 +213,9 @@ int main() {
 #define M8(x,y) (x)*(y)
   assert(63, M8(3+4, 4+5), "M8(3+4, 4+5)");
 
+#define M8(x,y) x y
+  assert(9, M8(, 4+5), "M8(, 4+5)");
+
   printf("OK\n");
   return 0;
 }