ci: handle no shell files in the Shellcheck check

.github/workflows/shellcheck-check.yaml

@@ -24,6 +24,7 @@ jobs:
         with:
           fetch-depth: 0
       - name: ShellCheck check
+        # https://www.shellcheck.net/wiki/Recursiveness
         run: |
           if find . -type f -name "*.sh" -print0 | grep -qz .; then
             find . -type f -name "*.sh" -print0 | xargs -0 shellcheck