[FeatureRequest] don't show NaN

[Santanachia]

Is your feature request related to a problem? Please describe. When the device is turned off, the last value is no longer visible

Describe the solution you'd like When the sensor is not available it should still show the last known value, and in the last updated field we should see how long it has not been updated. You could also add an mdi:access-point-network-off icon somewhere.