Only show collections for which at least one layer is configured

[Schpidi]

Alternatively also merge the BYOC collections for the default instance.

What is your opinion @constantinius @achtsnits ?

[constantinius]

In the client I think it is better to hide Datasets/BYOD-Collections when no layers are configured for them

For WMS/WCS Capabilities I think we should always show all datasets/collections regardless whether there are layers