ci: do not version bump on no changes

The script needs to exit if there are no changes produced by running `release-plz update`, otherwise
the `git commit` command will cause an error in the workflow.

.github/workflows/version_bump.yml

@@ -42,6 +42,10 @@ jobs:
         run: |
           set -e
           release-plz update
+          if [[ -z $(git status --porcelain) ]]; then
+            echo "No changes were detected. Exiting without bumping the version."
+            exit 0
+          fi
           version=$(cat Cargo.toml | grep "^version" | awk -F '=' '{ print $2 }' | xargs)
           commit_message="chore: release $version"
           git add --all