message: Fix mdbook.yml GitHub workflow not to fail when there is no change in /book
diff: @@ -37,5 +37,7 @@ jobs:
  run:
  cd gluesql.github.io &&
  git add book &&
  + git diff-index --quiet HEAD || (
  git commit -m "Update book!" &&
  git push https://panarch:${{ secrets.GLUESQL_ORG }}@github.com/gluesql/gluesql.github.io.git
  + )
  