// Bucket retrieves a bucket by name.
// Returns nil if the bucket does not exist.
// The bucket instance is only valid for the lifetime of the transaction.
func (tx *Tx) Bucket(name []byte) *Bucket {
	return tx.root.Bucket(name)
}

The comment says Returns nil if the bucket does not exist. but actually new bucket is created if it does not exist.

Update: did some more testing, it seems it returns a bucket when a new db is created and opened, and actually returns nil after the bucket is explicitly deleted using DeleteBucket(name)