Ensure report-built-images doesn't fail when no images were built

Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab/ci/report.yml

@@ -2,8 +2,19 @@ report-built-images:
   image: alpine
   stage: report
   script:
-    - cat *-built-image.txt > all-built-images.txt
-    - cat all-built-images.txt
+    - |
+      set -- "${CI_PROJECT_DIR}"/*-built-image.txt
+
+      while [ "$#" -gt 0 ] && [ ! -f "$1" ]; do
+        shift
+      done
+
+      if [ -f "$1" ]; then
+        cat *-built-image.txt > all-built-images.txt
+        cat all-built-images.txt
+      else
+        echo "No images built";
+      fi
   artifacts:
     paths: ["all-built-images.txt"]
     expire_in: 90d