Backport #40237 by @Shadow0Bit
Fixes#40230
Cookbooks without dependencies are served with `"dependencies": null`
from the universe and version metadata endpoints. This happens because
`omitempty` drops the empty map when the metadata is stored. Chef
clients iterate over this value and Policyfile resolution crashes.
Supermarket returns `{}` for these cookbooks, Gitea now does too. The
fix checks whether dependencies are `nil` while building response,
returns an empty map if true. This way cookbooks that are already stored
are covered without a re-upload.
Tested with the Chef integration test and `chef-cli install`.
Assisted by Claude Code (claude-opus-5-5).
Co-authored-by: rafał przypek <58443512+Shadow0Bit@users.noreply.github.com>
