Error out early when there are null entity ids in the cohort

Linked to issue #186. Currently, triage causes an error later in the pipeline when there are null entity_ids.

In the above issue, Tristan suggests adding this check into the experiment validator.

Read linked issue #186 first, then locate the experiment validator and trace where cohort entity_ids are validated. Confirm the validator rejects null entity IDs before triage reaches the later pipeline failure; done means the invalid cohort fails early with a clear error.

Written by the indexing model from the issue text.