Fix dumps with idle_view

[spacegaier]

Fixes #658

The access to media_position_updated_at failed and resulted in the card from rendering with the idle feature.

YAML setup (did not test without the vertical-stack but that should have no impact here either way).

type: vertical-stack
cards:
  - type: custom:mini-media-player
    entity: media_player.sonos_living_front
    idle_view:
      when_paused: true

[kalkih]

Thanks for the PR, made me realise the source issue _attr should never be undefined in updatedAt.
Closing this in favor for #667