[web] fix waiting for cdn upload on staging-main (#23729)

Co-authored-by: Jimmy Domagala-Tang <jimmy.domagala-tang@overleaf.com>
GitOrigin-RevId: fdf0f2c0b0b8c9e83a1ea6f6f71305f9ff822f1f

services/web/bin/cdn_upload

@@ -67,5 +67,7 @@ if [[ "$BRANCH_NAME" == "main" ]] || [[ "$BRANCH_NAME" == "staging-main" ]]; the
   fi
 
   wait "$pid_staging" # wait for staging upload to finish, wait(1) will exit if the upload failed
-  wait "$pid_production" # wait for production upload to finish (if started), wait(1) will exit if the upload failed
+  if [[ -n "$pid_production" ]]; then
+    wait "$pid_production" # wait for production upload to finish (if started), wait(1) will exit if the upload failed
+  fi
 fi