You can store nil values in bbolt. In ForEach, the only way to determine if a key is a nil value or bucket is by attempting to cast it as a bucket.

bbolt_nil_value.go

package main

import (
	"fmt"

	"go.etcd.io/bbolt"
)

var bucketName = []byte("MyBucket")

func main() {
	db, err := bbolt.Open("my.db", 0600, nil)
	if err != nil {
		panic(err)
	}
	defer db.Close()

	if err := db.Update(func(tx *bbolt.Tx) error {
		b, err := tx.CreateBucketIfNotExists(bucketName)
		if err != nil {
			return err
		}
		return b.Put([]byte("answer"), nil)
	}); err != nil {
		panic(err)
	}
	if err := db.View(func(tx *bbolt.Tx) error {
		b := tx.Bucket(bucketName)
		return b.ForEach(func(k, v []byte) error {
			fmt.Println(string(k), string(v))
			return nil
		})
	}); err != nil {
		panic(err)
	}
}