fix(ci): skip timing refresh without runs

[sd-db]

Resolves #

Description

A scheduled timing refresh can legitimately find no eligible integration-test artifacts while the workflow is waiting for a complete sample. Previously, that condition returned a failure and stopped the refresh workflow.

Automatic discovery now exits successfully when no runs yield timing data, allowing the next scheduled refresh to retry. Explicit --run-ids requests still fail when they produce no timings, preserving their validation behavior. The change adds regression coverage for the automatic zero-run path.

Checklist

• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt-databricks next" section. (not applicable: CI-only timing refresh behavior)

Verification

• hatch run pytest tests/unit/test_regenerate_timings.py -v
• hatch run pre-commit run --all-files

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.