fix(insights): Fix empty breakdown breaking historical trends

[webjunkie]

Problem

Fixes #22928 Fixes POSTHOG-1D9M

Changes

• fix breakdown value differently
• adjust test

How did you test this code?

Added test

[sentry-io[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: posthog/hogql_queries/insights/funnels/funnel_trends.py

Function	Unhandled Issue
_format_results	Cohort.DoesNotExist: Cohort matching query does not exist. posthog.tasks.tasks.process_... Event Count: 3
_format_results	TypeError: '<' not supported between instances of 'list' and 'str' posthog.tasks.tasks.... Event Count: 3

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