Fail the aggregate check on an empty result set

A `for` loop over an empty list runs zero iterations and exits zero, so if the
needs expression ever came back empty the gate would pass without having checked
anything. Cheap insurance against a status check that silently means nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: glentner

.github/workflows/tests.yml

@@ -73,6 +73,10 @@ jobs:
         run: |
           results="${{ join(needs.*.result, ' ') }}"
           echo "upstream job results: ${results}"
+          if [ -z "${results}" ]; then
+            echo "::error::no upstream job results; refusing to pass vacuously"
+            exit 1
+          fi
           for result in ${results}; do
             if [ "${result}" != "success" ]; then
               echo "::error::a required job reported '${result}'"