fix(packages): return empty Chef dependencies instead of null (#40237)

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).

tests/integration/api_packages_chef_test.go

@@ -345,6 +345,7 @@ nwIDAQAB
 		assert.Equal(t, "opscode", info.LocationType)
 		assert.Equal(t, setting.AppURL+root[1:], info.LocationPath)
 		assert.Equal(t, fmt.Sprintf("%s%s/cookbooks/%s/versions/%s/download", setting.AppURL, root[1:], packageName, packageVersion), info.DownloadURL)
+		assert.Equal(t, map[string]string{}, info.Dependencies)
 	})
 
 	type listCase struct {
@@ -463,6 +464,7 @@ nwIDAQAB
 		assert.Equal(t, packageVersion, result.Version)
 		assert.Equal(t, packageURL, result.Cookbook)
 		assert.Equal(t, fmt.Sprintf("%s/versions/%s/download", packageURL, packageVersion), result.File)
+		assert.Equal(t, map[string]string{}, result.Dependencies)
 	})
 
 	t.Run("Delete", func(t *testing.T) {