[ML] Anomaly explanations missing for rare detectors.

[walterra]

PR #142999 introduced anomaly explanations in the Anomaly Explorer's anomaly table. When using a rare detector, the section in the table's expanded row ends up empty:

However, there is some data available to be displayed:

We should update the AnomalyExplanationDetails component to consider this information.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)