Add --no-run-if-empty option to xargs command

Without this option, xargs will run the `gh pr close` command with
no arguments when `gh pr list` outputs no PRs. Adding this option
prevents the error that can result.

.github/workflows/update-main-source.yml

@@ -32,7 +32,7 @@ jobs:
 
         # close all existing PRs from branches starting with "auto-update"
         gh pr --repo "$GITHUB_REPOSITORY" list --search 'head:auto-update' --json 'headRefName' --jq '.[] | join("\n")' | \
-          xargs -n 1 gh pr --repo "$GITHUB_REPOSITORY" close
+          xargs --no-run-if-empty -n 1 gh pr --repo "$GITHUB_REPOSITORY" close
 
         # push changes
         git push --set-upstream origin "$BRANCH"