ci: check_suite_report reads gh's 'no artifact matches' as an error #2074

The first live Experimental report (#2072) includes: The job summaries could not be read: no artifact matches any of the names or patterns provided.

That run predates #2071, so it uploaded no summary artifacts, and the report should have said nothing about summaries. util/check_suite_report.py treats only no valid artifacts found to download as "no summaries". The gh run download --pattern installed on the runner prints the wording above instead, so a run with no summaries is reported as an error. This was one of #2071's unverified points. The fix is to treat both wordings as "no summaries".