OnDemandRequester.get_counts_by_specimen doesn't error if none of the cells selected are in a specimen it looks at

[CarlinLiao]

If the set of cells_selected passed to OnDemandRequester.get_counts_by_specimen does not contain any cells for one of the specimens it looks at, it'll error. This is a common occurrence because, in the case of importance scores which uses this function, we often exclude certain specimens from evaluating with the CG-GNN because they're from an irrelevant cohort. get_counts_by_specimen needs to be refactored to handle this gracefully.