fix: return empty flat map when container appstruct is null

[santhreal]

Sequence.flatten hits TypeError when missing=drop sequence absent from appstruct. This PR fixes the regression with a focused test covering the case.

[SashankBhamidi]

See collective/icalendar#1508 (reply in thread)

[santhreal]

@stevepiercy Do you see an option to close this PR?

You could literally just ask to close the pr. Get off the high horse.

[SashankBhamidi]

You could literally just ask to close the pr. Get off the high horse.

Oh, thanks mate. Much appreciated.

[SashankBhamidi]

Hi, I just realised this is the wrong repo I commented in. Apologies.

[stevepiercy]

You could literally just ask to close the pr. Get off the high horse.

@santhreal your comment is inconsiderate and disrespectful. Please read and follow https://pylonsproject.org/community-code-of-conduct.html.

@SashankBhamidi owned up to his mistake. I'd advise you do the same. Thank you for your consideration.

[santhreal]

You could literally just ask to close the pr. Get off the high horse.

@santhreal your comment is inconsiderate and disrespectful. Please read and follow https://pylonsproject.org/community-code-of-conduct.html.

@SashankBhamidi owned up to his mistake. I'd advise you do the same. Thank you for your consideration.

I apologize, @SashankBhamidi. Sorry for the rude comment.

[stevepiercy]

@santhreal thank you for apologizing.

Would you like to reopen your pull request?

I'd also suggest making all tests pass in the CI checks. Right now there are failures, and backlogged maintainers are highly unlikely to review closed and failing PRs.