Don't allow creation of empty bands array in load_collection (and other places)

[dthiex]

Currently it's possible to created bands: [] with the editor resulting into a not valid PG being send to the backend.

I think it either shouldn't be allowed to construct an empty array with the editor or set it to null in such a case.

[m-mohr]

Strictly speaking, an empty bands array is allowed/valid in load_collection, the schema doesn't set minItems to 1:
https://github.com/Open-EO/openeo-processes/blob/draft/load_collection.json#L164
So I'm moving this over to openeo-processes so that we can fix it there. Once minItems is set to 1, the Editor should handle it correctly (but I'll verify it after fixing it here).