fix: add error message for empty min_lock_date filter

Author: akselthomsen

.github/workflows/check_nn_versions.yaml

@@ -62,6 +62,11 @@ jobs:
         id: filter
         run: |
           echo "config=$(jq --arg date $MIN_LOCK_DATE --compact-output '[.[] | select(.date >= $date)]' config.json)" >> "$GITHUB_OUTPUT"
+      - name: Validate filtered versions
+        if: steps.filter.outputs.config == '[]'
+        run: |
+          echo "::error::No R versions match min_lock_date '${{ env.MIN_LOCK_DATE }}'. Choose a date before the earliest configured version."
+          exit 1
   R-CMD-check:
     needs: filter-nn-versions
     runs-on: '${{ matrix.os }}'