makeDecl for a nil value needs to use the original type

[shueybubbles]

This code path in makeParam is being hit because CheckNamedValue sends the sql.Nullable* type instances to convertInputParameter

func convertInputParameter(val interface{}) (interface{}, error) {
	switch v := val.(type) {
	case int, int16, int32, int64, int8:
		return val, nil
	case byte:
		return val, nil
	case VarChar:
		return val, nil
	case NVarCharMax:
		return val, nil
	case VarCharMax:
		return val, nil
	case NChar:
		return val, nil
	case DateTime1:
		return val, nil
	case DateTimeOffset:
		return val, nil
	case civil.Date:
		return val, nil
	case civil.DateTime:
		return val, nil
	case civil.Time:
		return val, nil
	// case *apd.Decimal:
	// 	return nil
	case float32:
		return val, nil
	default:
		return driver.DefaultParameterConverter.ConvertValue(v)
	}
}

The DefaultParameterConverter just returns nil so we lost the type information. If the value implements driver.Valuer it should be preserved when it has a nil value so makeParam can use it.

func (s *Stmt) makeParam(val driver.Value) (res param, err error) {
	if val == nil {
		res.ti.TypeId = typeNull
		res.buffer = nil
		res.ti.Size = 0
		return
	}
...
func makeDecl(ti typeInfo) string {
	switch ti.TypeId {
	case typeNull:
		// maybe we should use something else here
		// this is tested in TestNull
		return "nvarchar(1)"

makeParam should