HTML reports tables fail if no tags or jobs

Current situation

The following insight causes an internal error if there is no matching testcases:

- name: junit-summary-for-test-suite
  kind: SummaryReport
  spec:
    scope: test.technology == 'junit'
    template:
    - name: tags_table
      kind: Table
      datasource: tags

Desired outcome

No internal error (and more debug info).

The bug is caused by the following piece of code:

if not _has_testcases(item.get('datasource'), source):
    warnings.add(
        f'No test cases matching scope `{item["scope"]}` for {item["kind"]} "{item["name"]}", generated item may contain no data.'
    )