godrv: Get a nil interface{} if a column field of type DATETIME is null

Previously, if you had a nil interface{} or *time.Time as a scan destiny, and your date/time result was null, godrv created a zero time.Time. This is inconsistent with both mymysql/native and database/sql behavior, and makes impossible to tell a zero value from a null value.

With this little change null field -> time.Time still makes a zero time.Time, but null field -> *time.Time, interface{} will result in a nil pointer/interface value.