cgen: fix optional struct declarations for empty structs (#13970)

vlib/json/json_decode_with_optional_arg_test.v

@@ -1,9 +1,7 @@
 import json
 import os
 
-struct DbConfig {
-	foo int
-}
+struct DbConfig {}
 
 fn test_json_decode_with_optional_arg() {
 	if ret := print_info() {

vlib/v/gen/c/cgen.v

@@ -972,7 +972,7 @@ fn (g Gen) optional_type_text(styp string, base string) string {
 	ret := 'struct $styp {
 	byte state;
 	IError err;
-	byte data[sizeof($size)];
+	byte data[sizeof($size) > 0 ? sizeof($size) : 1];
 }'
 	return ret
 }