json: fix [omitempty] with string (#17813)

vlib/json/json_omitempty_test.v

@@ -0,0 +1,25 @@
+import json
+
+pub struct MyStruct {
+pub mut:
+	code    int
+	message string
+	data    string  [omitempty]
+	data2   ?string [omitempty]
+}
+
+fn test_simple() {
+	obj := MyStruct{
+		code: 1
+		message: 'yes'
+		data2: 'a'
+	}
+	assert dump(json.encode(obj)) == '{"code":1,"message":"yes","data2":"a"}'
+}
+
+fn test_none() {
+	obj := MyStruct{
+		code: 1
+	}
+	assert dump(json.encode(obj)) == '{"code":1,"message":""}'
+}