cgen: fix json encode struct with optional field (#16866)

vlib/json/json_encode_struct_with_optional_field_test.v

@@ -0,0 +1,20 @@
+import json
+
+struct Foo {
+	name string
+	num  ?int
+}
+
+fn test_json_encode_struct_with_optional_field() {
+	f1 := Foo{
+		name: 'hello'
+	}
+	ret1 := json.encode(f1)
+	println(ret1)
+	assert ret1 == '{"name":"hello","num":null}'
+
+	f2 := Foo{'hello', 22}
+	ret2 := json.encode(f2)
+	println(ret2)
+	assert ret2 == '{"name":"hello","num":22}'
+}