In the SPARQL queries that integraality uses to find groupings, no difference is made between the lack of value and a set to "No value"

I’m not sure whether this is actually an issue and if anyone cares. Are there any use cases to have a slicing over "No value"?

(It’s probably doable to support this, but the naming will be interesting as we already have the "no-grouping" option.)

Relates somewhat to T273226