json: fix option state (#18802)

vlib/json/json_option_none_test.v

@@ -0,0 +1,46 @@
+import json
+
+struct Struct {
+	a int
+}
+
+struct Test {
+	a ?int
+	b ?string
+	c ?Struct
+}
+
+fn test_main() {
+	a := json.decode(Test, '{"a": 1, "b": "foo"}')!
+	dump(a)
+
+	assert a.a != none
+	assert a.b != none
+
+	b := json.decode(Test, '{"a": 1}')!
+	dump(b)
+	assert b.a != none
+	assert b.b == none
+
+	c := json.decode(Test, '{"a": 1, "b": null}')!
+	dump(b)
+	assert c.a != none
+	assert c.b == none
+
+	d := json.decode(Test, '{"a": null, "b": null}')!
+	dump(d)
+	assert d.a == none
+	assert d.b == none
+
+	e := json.decode(Test, '{"a": null, "b": null, "c": null}')!
+	dump(e)
+	assert e.a == none
+	assert e.b == none
+	assert e.c == none
+
+	f := json.decode(Test, '{"a": null, "b": null, "c": {"a":1}}')!
+	dump(f)
+	assert f.a == none
+	assert f.b == none
+	assert f.c != none
+}