bug: Integration test matrix fails when no itests exist

We found [this error](https://github.com/canonical/cos-proxy-operator/actions/runs/14472690856) in cos-proxy which states:
> Error when evaluating 'strategy' for job 'integration-test'. canonical/observability/.github/workflows/_charm-quality-checks.yaml@v1 (Line: 250, Col: 16): Matrix vector 'suite' does not contain any values

Cos-proxy [has no itest folder](https://github.com/canonical/cos-proxy-operator/tree/main/tests) so the [suite var is empty](https://github.com/canonical/observability/blob/main/.github/workflows/_charm-quality-checks.yaml#L250).

Start with .github/workflows/_charm-quality-checks.yaml at line 250 and inspect how the integration-test job builds its suite matrix. Reproduce the workflow with a repository that has no tests/itest folder, then verify the workflow succeeds without an empty matrix while preserving integration-test execution when suites exist.

Written by the indexing model from the issue text.