Fix UX/JS/caching/PHP issues caused by #3479245

Problem/Motivation

#3479245: JS Warning About Not Enough Slides introduced many problems:
* PHP warning because of sometimes `count()` on NULL.
* Despite the permissions check, this patch doesn't set cache contexts, so the warning message will be cached and shown to anonymous visitors.
* I'm unsure why a content editor needs to be told that a widget is choosing option B instead of option A. This is just confusion inducing.
* IMO the JS console message is a _good_ thing to help debug what's going on. IMO there's no need to remove it in the first place, though I understand why others might disagree.

Steps to reproduce

Create content with only one or zero items in the Swiper.

Proposed resolution

Adjust `template_preprocess_swiper_formatter()` to fix problems 1–3. We can leave 4 as-is.

Remaining tasks

review and commit

User interface changes

no more warning message about only 1 item

API changes

none

Data model changes

none