Fix NULL values returned as driver.Value

[fiorix]

The current typeConverter.ConvertValue function always return the default value for types, as opposed to nil, which causes types such as sql.Null* to always have Valid=true even when the value returned by the query is NULL.

e.g.

// ConvertValue implements the driver.ValueConverter interface.
func (c *typeConverter) ConvertValue(v interface{}) (driver.Value, error) {
	switch c.parsedType[0] {
	case "boolean":
		vv, err := scanNullBool(v)
		return vv.Bool, err

The last line causes the returned sql.NullBool to always be valid. In order to fix, we have to:

if !vv.Valid { return nil, err }

Our integration tests for type conversion should test this. I found this by accident.