Harden artifact staging loop and clarify PR metadata var

.github/workflows/build.yml

@@ -65,18 +65,18 @@ jobs:
             BASE_VERSION="0.9.1"
           fi
           SHORT_SHA="$(git rev-parse --short=8 "${HEAD_SHA}")"
-          PR_NUMBER_SAFE="${PR_NUMBER:-0}"
+          PR_NUMBER_OR_ZERO="${PR_NUMBER:-0}"
 
           if [ "${{ github.event_name }}" = "pull_request" ]; then
-            VERSION="${BASE_VERSION}-pr${PR_NUMBER_SAFE}+${SHORT_SHA}"
-            ARTIFACT_VERSION="${BASE_VERSION}-pr${PR_NUMBER_SAFE}-${SHORT_SHA}"
+            VERSION="${BASE_VERSION}-pr${PR_NUMBER_OR_ZERO}+${SHORT_SHA}"
+            ARTIFACT_VERSION="${BASE_VERSION}-pr${PR_NUMBER_OR_ZERO}-${SHORT_SHA}"
           else
             VERSION="${BASE_VERSION}"
             ARTIFACT_VERSION="${BASE_VERSION}-${SHORT_SHA}"
           fi
 
           echo "base_version=$BASE_VERSION" >> "$GITHUB_OUTPUT"
-          echo "pr_number=$PR_NUMBER_SAFE" >> "$GITHUB_OUTPUT"
+          echo "pr_number=$PR_NUMBER_OR_ZERO" >> "$GITHUB_OUTPUT"
           echo "short_sha=$SHORT_SHA" >> "$GITHUB_OUTPUT"
           echo "artifact_version=$ARTIFACT_VERSION" >> "$GITHUB_OUTPUT"
           echo "version=$VERSION" >> "$GITHUB_OUTPUT"
@@ -126,6 +126,7 @@ jobs:
           ARTIFACT_VERSION: ${{ steps.build_version.outputs.artifact_version }}
         run: |
           set -eu
+          shopt -s nullglob
           mkdir -p build/artifacts
 
           for f in build/openpuck/*.{uf2,bin,hex,elf,map,zip}; do