Publish workflow fails when no changes to api docs between versions

[ml-evs]

Just ran into this at https://github.com/Materials-Consortia/optimade-python-tools/actions/runs/488548054.

If the API docs have no updates between versions (as in 0.12.7) then the changes to the changelog will cause the workflow to try and make an empty commit. The line below does not filter out the case where CHANGELOG has been modified (by the workflow) but API docs have no changes:

optimade-python-tools/.github/workflows/update_docs.sh

Line 13 in b4d1b3d

if [ -n "$(git status --porcelain --untracked-files=no --ignored=no)" ]; then