NULL interval comes back as zero time.Time

[obno]

Non NULL INTERVAL DAY TO SECOND data types come back as time.Durations as expected. However, their NULL counterparts come back as zero time.Times making the interface inconsistent.

Looks like https://github.com/go-goracle/goracle/blob/master/rows.go#L416 should set to nil.

[tgulacsi]

Thanks!