json: fix json with option struct (#17942)

vlib/json/json_option_struct_test.v

@@ -0,0 +1,23 @@
+import json
+
+pub struct SomeStruct {
+pub mut:
+	test ?string
+}
+
+pub struct MyStruct {
+pub mut:
+	result ?SomeStruct
+	id     string
+}
+
+fn test_main() {
+	a := MyStruct{
+		id: 'some id'
+		result: SomeStruct{}
+	}
+	encoded_string := json.encode(a)
+	assert encoded_string == '{"result":{},"id":"some id"}'
+	test := json.decode(MyStruct, encoded_string)!
+	assert test == a
+}