fix(trace-explorer): Handle empty baseline cohort in ranked attributes API

[seer-by-sentry]

Surfaced from SENTRY-555Z, but does not directly address its root cause

The /api/0/organizations/{organization_id_or_slug}/trace-items/attributes/ranked/ endpoint was returning a 500 error when the calculated total_baseline was zero or negative.

This occurred because the downstream Seer service would encounter a ZeroDivisionError when attempting Laplace smoothing with a total_count of zero. The total_baseline in Sentry becomes zero when the "outlier" cohort query matches all or more spans than the "baseline" cohort query.

This change introduces an early return in the Sentry endpoint. If total_baseline <= 0, the endpoint now returns a 200 OK response with an empty rankedAttributes list. The response shape now consistently includes rankingInfo, cohort1Total, and cohort2Total even in this early return case, providing a more complete and predictable API response to the frontend.

This prevents 500 errors for users, provides a consistent API response, and avoids unnecessary calls to the Seer service when no meaningful comparison can be made.

closes AIML-2306: c.w. endpoint bug

[linear]

AIML-2306 c.w. endpoint bug