json: fix json decode/encode with option type (#17393)

vlib/json/json_option_test.v

@@ -0,0 +1,68 @@
+import json
+
+pub struct MyStruct {
+pub mut:
+	valuea int
+}
+
+pub struct MyStruct2 {
+pub mut:
+	valuea int
+	valueb ?MyStruct
+}
+
+struct Node {
+	location NodeLocation [json: 'loc']
+}
+
+struct NodeLocation {
+	source_file ?SourceFile [json: 'includedFrom']
+}
+
+struct SourceFile {
+	path string [json: 'file']
+}
+
+fn test_encode_decode() {
+	assert json.encode(MyStruct2{ valuea: 1 }) == '{"valuea":1}'
+
+	assert json.decode(MyStruct2, '{"valuea": 1}')! == MyStruct2{
+		valuea: 1
+		valueb: none
+	}
+}
+
+fn test_encode_decode2() {
+	assert json.encode(MyStruct2{ valuea: 1, valueb: none }) == '{"valuea":1}'
+
+	assert json.decode(MyStruct2, '{"valuea": 1}')! == MyStruct2{
+		valuea: 1
+		valueb: none
+	}
+}
+
+fn test_encode_decode3() {
+	assert json.encode(MyStruct2{
+		valuea: 1
+		valueb: MyStruct{
+			valuea: 123
+		}
+	}) == '{"valuea":1,"valueb":{"valuea":123}}'
+
+	assert json.decode(MyStruct2, '{"valuea": 1}')! == MyStruct2{
+		valuea: 1
+		valueb: none
+	}
+}
+
+fn test_main() {
+	node := json.decode(Node, '{"loc": { "includedFrom": { "file": "/bin/foo" } } }')!
+
+	source_file := node.location.source_file or {
+		SourceFile{
+			path: '-'
+		}
+	}
+
+	assert source_file.path == '/bin/foo'
+}