The "/statistics" endpoint returns empty result for non-WGS84 grid mappings

[forman]

Describe the bug

Calling the "/statistics" endpoint of xcube server for datasets with a non-WGS84 grid mapping will return an empty result (NaN).

To Reproduce

Steps to reproduce the behavior:

1. Start xcube server with demo configuration
2. http://localhost:8080/viewer
3. Enable the map's info box
4. Switch to Water quality cube example (geogr., WGS84) --> OK
5. Goto https://open-sr.api.brockmann-consult.de/api/viewer/
6. Enable the map's info box
7. Switch to "Nordfriesland" cube (UTM) --> Empty

The same applies to the statistics panel.

Screenshots