Check the registry entry in sensor unit_of_measurement instead of unique_id

[bdraco]

Proposed change

The unit_of_measurement check was checking to see if the entity has a unique_id instead of a registry entry. Its much cheaper to check for the registry_entry than the unique id since some entity have to construct it every time its read

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Hey there @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (sensor) you are listed as a code owner for? Thanks!

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[emontnemery]

It'd be really nice if we could cache properties, the unique_id is an example of a property which can't possibly change.
Still, nice cleanup.

[bdraco]

It'd be really nice if we could cache properties, the unique_id is an example of a property which can't possibly change. Still, nice cleanup.

I think we can after #95315 but thats blocked by #95407

[bdraco]

thanks