fix: make sure umap.properties.slideshow is defined

[yohanboniface]

No description provided.

[yohanboniface]

Not sure why it fails while it is said as "widely available" here:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment

[davidbgk]

We need to bump to 2021 here (cf https://stackoverflow.com/questions/76092858/how-can-i-get-eslint-to-accept-the-nullish-coalescing-assignment)

[davidbgk]

I check and npx eslint umap/static/umap/js/ doesn't propose anything better with the new baseline, I guess we'll have to figure it out by ourselves 😬