feat(encoding/gjson): add OmitEmpty option to respect omitempty tag

[lingcoder]

Summary

• Add OmitEmpty option to gjson.Options struct
• Pass OmitEmpty value to gconv.Map() instead of hardcoding false
• Add unit tests to verify the new option

Background

When using gjson.New() to convert a struct to JSON, fields with omitempty tag were not being omitted even when their values were empty. This was inconsistent with the standard library json.Marshal() behavior.

Before:

type User struct {
    Age    int `json:"age,omitempty"`
    Height int `json:"height,omitempty"`
}
user := User{Age: 1}
gjson.New(user).String() // {"age":1,"height":0}  - height not omitted
json.Marshal(user)       // {"age":1}             - height omitted

After:

gjson.NewWithOptions(user, gjson.Options{OmitEmpty: true}).String() // {"age":1}

Test plan

• Add unit test Test_NewWithOptions_OmitEmpty covering:
  • Default behavior (OmitEmpty: false) - empty fields included
  • With OmitEmpty: true - empty fields omitted
  • All empty values scenario

Closes #4450

[lingcoder]

还是放弃修改了，基础库先不动，影响面比较大