Fix calendar parsing on from_str implementation

[nekevss]

parse_allowed_calendar_formats was returning Some(None) instead of None. This was preventing calendar ids from correctly moving to Calendar::from_utf8

[jedel1043]

Uhhh can you remind me again why we have Option<Option<&[u8]>>? There are too many empty values there:

• None
• Some(None)
• Some(Some(&[]))

Do we use all of them?

[nekevss]

Hmmmm, we're swallowing the error here from the ParseCalendarString.

So the return type could be Result<Option<&[u8]>, but we are swallowing the error anyways in this instance, so it doesn't offer much utility to return an Error. As it currently stands, the Some(None) return was incorrectly signalling a normal completion on the format parsing and going down the wrong code path.

EDIT: The question on Some(Some(&[])) is interesting. I believe those are rejected by Calendar::from_utf8 (Plus I believe [u-ca=] should be an invalid annotation)

[nekevss]

Update: I was able to confirm the ixdtf does correctly throw an error on [u-ca=], so the empty value case of Some(Some(&[])) is handled during parsing.

Even on the cases that it is not handled, it would be rejected in Calendar::from_utf8.

[jedel1043]

@nekevss In that case, can we flatten the Options and use &[] as our empty value instead?

[nekevss]

can we flatten the Options and use &[] as our empty value instead?

Sure! Actually, I think I like this better. Added a sanity test too for empty calendar annotations.

[jedel1043]

Looks good!