fix(modules/*): filter out falsy elements

[jordanrastrick]

N.B. I have not linked this PR to an issue because I can't provide my employer's application as a demo, and it would take vastly more time to strip it down to a working reproduction that I could get permission to share, than it would for the validity of the code fix to be assessed.

Under some circumstances when the the a11y module calls destroy() in response to receiving a destroy event, it throws an exception (which can't be easily handled due to the asynchronicity).

When the elements are retrieved from the swiper instance's navigation property:

let { nextEl, prevEl } = swiper.navigation ? swiper.navigation : {};

evidently they are arrays containing undefined.

The conservative change I have made and which I have validated fixes the problem is to extend the existing truthiness filter in makeElementsArray for the single element case to the case when the input is already an array.

[nolimits4web]

Merged, thank you!