Support omitempty option on JSON marshaling

[moznion]

This pull request makes it support the omitempty option on JSON marshaling. If the property has that option and the value is None[T], it omits that property from serialized JSON string.

example:

type JSONStruct struct {
	OmitemptyVal Option[string] `json:"omitemptyVal,omitempty"` // this should be omitted
}
jsonStruct := &JSONStruct{OmitemptyVal: None[string]()}
marshal, err := json.Marshal(jsonStruct)
if err != nil {
	return err
}
fmt.Printf("%s\n", marshal) // => {}