json: fix option sumtype with int types (#18013)

vlib/json/json_encode_sumtype_test.v

@@ -0,0 +1,13 @@
+import json
+
+struct Test {
+	optional_sumtype ?MySumtype
+}
+
+type MySumtype = int | string
+
+fn test_simple() {
+	test := Test{}
+	encoded := json.encode(test)
+	assert dump(encoded) == '{}'
+}