Show thumbnail if map is empty

[m-mohr]

Suggestion from @ycespb

When a STAC item has a geometry which is “NULL”, and when a “preview” image is provided. Would it be possible to show by default the “preview” instead of the empty map ? [...] This would be very nice for items without geometry.

Examples:
https://radiantearth.github.io/stac-browser/#/external/emc.spacebel.be/collections/services/items/ai4eo-neuromorphic-computing?httpAccept=application%2Fgeo%2Bjson%3Bprofile%3Dhttps%3A%2F%2Fstacspec.org
More are available in the subcatalogs below https://radiantearth.github.io/stac-browser/#/external/emc.spacebel.be/concepts/tools/5881fd91-58e6-58cb-a4f3-014ae246c67b

I tested this today and there is no reliable way right now to detect how many layers are shown on the map as stac-layer 0.15.0 seems to also add layers that are empty.