`LatestValueCache` can't use `None` as a valid value

[llucax]

What happened?

LatestValueCache is using None internally to represent the lack of a value, but None is also a valid value.

What did you expect instead?

LatestValueCache to use a sentinel to represent a missing value instead.

Affected version(s)

No response

Affected part(s)

Core components (data structures, etc.) (part:core)

Extra information

Introduced here:

• Un-Peekable-ification of the PowerDistributingActor #800 (comment)