DynamicBackground: fallback to set thumbnail
DynamicBackground: fallback to set thumbnail

Noticed that, when the set page has no articles (e.g. only
subsets), the background shows the default gray background.
This is easier to see now that we show the subsets previews
on top of the articles. Therefore, we could fallback to set
thumbnail instead.

https://phabricator.endlessm.com/T13191

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib