The DLNA device list endpoint could return {"devices":null} in two cases:
the SSDP-failure path returned a nil slice, and the cached path copied an
empty cache via append([]DLNADevice(nil), cache...) which also yields nil.
The web UI reads res.devices.length, so a null devices field crashed the
DLNA page to a white screen. Return non-nil slices in both paths and guard
the frontend with a null coalesce. Adds a regression test.
Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
soldosluka857