ci: Do not fail when there is nothing to commit

When there is nothing to commit ci will fail with:

nothing to commit, working tree clean
Error: Process completed with exit code 1.

This was reported as #287.
Catch this by checking via `git diff-index` if there is anything to commit first.

.github/workflows/update-themes.yml

@@ -38,5 +38,5 @@ jobs:
           git config --global user.email "bep@users.noreply.github.com"
           git config --global user.name "bep"
           git add .
-          git commit -am "[Bot] Update themes"
+          git diff-index --quiet HEAD || git commit -am "[Bot] Update themes"
           git push --force-with-lease