To use `sql.NullString` or `sql.NullInt64` on a Go (golang) struct you need to wrap sql.NullString inside a new type, then implement the json.Unmarshaler interface on that new type.

golang_null_field.go

package main

import (
	"database/sql"
	"encoding/json"
	"fmt"
	"log"
	"strings"
)

type NullString sql.NullString

func (s *NullString) UnmarshalJSON(data []byte) error {
	s.String = strings.Trim(string(data), `"`)
	s.Valid = true
	return nil
}

type SendgridEvent struct {
	Email       string     `json:"email"`
	Timestamp   int        `json:"timestamp"`
	SMTPID      string     `json:"smtp-id,omitempty"`
	SgEventID   string     `json:"sg_event_id"`
	SgMessageID string     `json:"sg_message_id"`
	Event       string     `json:"event"`
	IP          string     `json:"ip,omitempty"`
	URL         string     `json:"url,omitempty"`
	Useragent   NullString `json:"useragent,omitempty"`
	SgUserID    int        `json:"sg_user_id,omitempty"`
	AsmGroupID  int        `json:"asm_group_id,omitempty"`
}

func main() {

	body := `[
    {
      "email":"[email protected]",
      "timestamp": 1337197600,
      "smtp-id":"<[email protected]>",
      "sg_event_id":"sendgrid_internal_event_id",
      "sg_message_id":"sendgrid_internal_message_id",
      "event": "processed"
    },
    {
      "email":"[email protected]",
      "timestamp": 1337966815,
      "ip":"X.XX.XXX.XX",
      "sg_event_id":"sendgrid_internal_event_id",
      "url":"https://sendgrid.com",
      "sg_message_id":"sendgrid_internal_message_id",
      "useragent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36",
      "event":"click"
    },
    {
      "ip": "X.XX.XXX.XX",
      "sg_user_id": 123,
      "sg_event_id":"sendgrid_internal_event_id",
      "sg_message_id":"sendgrid_internal_message_id",
      "useragent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36",
      "event": "group_unsubscribe",
      "email": "[email protected]",
      "timestamp": 1337969592,
      "asm_group_id": 42
    }
  ]`
	r := strings.NewReader(body) // fake request body

	var records []SendgridEvent
	err := json.NewDecoder(r).Decode(&records)

	if err != nil {
		log.Println(err)
		return
	}

	for _, r := range records {
		fmt.Printf("%+v\n", r)
	}

}