inverted logic in EpochTime

[sbinet]

in epoch_time.go, one can see:

openfoodfacts-go/epoch_time.go

Line 25 in 80078b8

secs, err := strconv.ParseInt(string(b), 0, 64)

	secs, err := strconv.ParseInt(string(b), 0, 64)
	if err == nil {
		t.Time = time.Time{}
		return nil
	}

• the condition in the if should probably be err != nil
• one should probably return the err error as well (instead of nil)