Support for 'some value' / 'unknown value' / 'no value' types of values in Wikibase statements has been requested a while ago (see GitHub issue). Some time ago a partial approach/solution for this issue has already been developed. I have noticed that there's not much awareness of this partial solution, and if folks know that it's possible, they can't find documentation on how to currently do some value (unknown value) / no value statements in Wikibases.
To hopefully make this partial approach more visible and the approach discoverable, here's a demo video I just made.
GitHub issue for those who want to contribute to the rest of the solution: Add "no value" and "unknown value" as possible values for a Wikidata statement · Issue #1713 · OpenRefine/OpenRefine · GitHub