proposal: database/sql: add NullInt16 support

[kyleconroy]

Hey Go team!

I maintain a database code generator. I'm having trouble correctly supporting null smallint columns because the database/sql package does not have a NullInt16 type. It would look almost identical to Nullint32 and NullInt64.

// NullInt16 represents an int32 that may be null.
// NullInt16 implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullInt16 struct {
	Int16 int16
	Valid bool // Valid is true if Int16 is not NULL
}

// Scan implements the Scanner interface.
func (n *NullInt16) Scan(value interface{}) error {
	if value == nil {
		n.Int16, n.Valid = 0, false
		return nil
	}
	n.Valid = true
	return convertAssign(&n.Int16, value)
}

// Value implements the driver Valuer interface.
func (n NullInt16) Value() (driver.Value, error) {
	if !n.Valid {
		return nil, nil
	}
	return int64(n.Int16), nil
}

[ianlancetaylor]

CC @kardianos