sync: fix error of empty struct channel (fix #17556) (#17597)

vlib/sync/empty_struct_chan_init_test.v

@@ -0,0 +1,8 @@
+struct User {
+}
+
+fn test_empty_struct_chan_init() {
+	user_ch := chan User{cap: 10}
+	println(user_ch)
+	assert true
+}