Web UI No Longer Breaks on Missing Telemetry Fields

PX4 reports an unavailable float field — for example, airspeed with no pitot sensor installed — as NaN. A single NaN field could stop Horizon UI’s status stream and make the web map unreachable. All float-sourced telemetry is now guarded at ingestion: an unavailable field reports as unknown rather than breaking the feed, and GPS-reported altitude is preserved when the flight controller’s own altitude field is unavailable.