[FR] Ignore missing entities

[AleXSR700]

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If an entity is missing (e.g. device is temporarily not connected to HA due to maintenance), the card stops working because it fails to load that entity.

Only way of getting the card working again is to remove all entities that are currently unavailable to HA. That, of course, is a lot of work because usually they are children of at least one parent.

But the alternative right now is a broken card that of course throws errors and messes up the dashboard :(

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Ignore missing entities. This is also implemented in the energy dashboard, probably for the exact same reason.

If an entity is no longer available, HA Energy integration ignores that entity for periods where it is not available but shows it for past periods where it was available.
This allows the user to see old devices and their contribution even if they are no longer available now.

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