fix(logring): inline empty-key groups

[dhodhoo]

Description

logring currently adds a separator when flattening an empty-key slog.Group under an existing group prefix. This can produce malformed retained fields such as:

svc..status=200

instead of:

svc.status=200

The slog.Handler contract specifies that groups with an empty key should have their attributes inlined.

This change keeps the existing prefix unchanged for empty-key groups while preserving the current behavior for named groups.

Regression coverage includes empty groups under a named prefix, nested empty groups, and consecutive empty groups.

No related issue; this was found while reviewing the structured log flattening path.

Type of change

• Bug fix
• New feature
• Refactor / internal
• Docs only
• Dependency update

Checklist

• Code builds: go build ./...
• go vet ./... clean
• Tests pass: go test ./... (CI also runs -race)
• Tests added/updated for the change
• Public docs updated if behavior or config changed (README.md, docs/guides/) — not applicable; no public config or documentation changes
• No secrets: no real tokens, .env, or config.json content

Notes

Targeted logring tests pass with the race detector, and the full local race suite passes sequentially with go test -race -p 1 ./....