Durations without units aren't valid durations

[Julian]

Hi! (Thanks for the library).

I believe the below isn't valid under the spec:

isoduration.parse_duration("P1")

which should be invalid, as it has no unit, but instead returns:

Duration(DateDuration(years=Decimal('0'), months=Decimal('0'), days=Decimal('0'), weeks=Decimal('0')), TimeDuration(hours=Decimal('0'), minutes=Decimal('0'), seconds=Decimal('0')))

#21 is possibly related.