Handle no posts without crashing

[deBhal]

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Changes proposed in this Pull Request:

I was testing the Carousel on an atomic site and ran in a series of weird errors - a missing element that swiper expects to exist, a bunch of this referring to the class instead of the instance, and a couple of unsafe dereferences when there are no slides.

This still feels like it might be me doing something wrong, but I wanted to get these changes up to see if someone could explain why I'm seeing them, so a PR isn't quite right, but it's pretty close.

I'm seeing this on an ephemeral site where I'm uploading the synced-coblocks plugin manually, and in chrome.

Closes # .

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[adekbadek]

Hi @deBhal , sorry for the wait. Could you please provide steps to reproduce the issue(s)? Also, no-posts-handling was improved in #731, maybe it resolves the issue.