backend-drm,pipewire,remoting: do not set monitor serial to "unknown"

If serial is unknown, it's best to leave it as NULL. All usage sites
already deal with it possibly being NULL.

This makes DRM-backend consistent with all other backend that leave
serial as NULL, allowing to move the initialization of these fields into
core.

Pipewire and remoting plugins are modified just for consistency.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>