Observer should not raise errors for workflows without testResult notifications

Current situation

When a workflow contains no testResult notifications, observer throws an error message ("No test results found for workflow {workflow_id}, can not generate datasource."). InsightCollector tries to generate default reports for this workflow, making numerous observer calls: it makes logs unclean.

Desired outcome

Having workflows without testResults is normal, so no error message should be generated for such workflows by observer. For a workflow without testcases, observer should return return an OK 200 response with empty details.items list. Callers (insightcollector, qualitygate, et al.) should correctly handle this response.