If an entity has no statements, then we don’t have any `CheckResults` to store in the result array (not even `NullResults`), so `CheckingResultsBuilder` returns a completely empty array. This is already a violation of the `wbcheckconstraints` API, I think (consumers should probably be able to expect the entity ID and an empty `claims` array, at least), but it also results in an error in `CachingResultsBuilder` when trying to access the results for that entity to cache them:
Notice: Undefined index: Q50580715 in /srv/mediawiki/php-1.31.0-wmf.25/extensions/WikibaseQualityConstraints/src/Api/CachingResultsBuilder.php on line 242
Not sure what the best solution for this will be… probably some kind of `NullContextCursor` (only entity ID, no statement etc.) that we can plug into the `NullResult` we already have from T178160?