fix(tracing): omit http.route span attribute when route has no paths

[officialasishkumar]

Summary

Per the OpenTelemetry semantic conventions, the http.route attribute must not be set on the span when the route is not known. Previously, Kong emitted an empty string as the value of http.route when the matched route had no paths defined (e.g., host-only routes), violating the specification.

This change makes http.route only be set when the route has an actual path value. When no path is available, the attribute is omitted entirely.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/developer.konghq.com - N/A (internal behavioral fix to match existing OTel spec)

Issue reference

Fix #14845

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