State_with_unit: do not show uom if unavailable/unknown

[Ildar_Gabdullin]

Currently “state_with_unit” shows a UoM when a sensor is “unavailable”:

This is why I am not using “state_with_unit”, have to compose an output dependently on a current state.
Suggest to not add UoM if a state is “unavailable”. Probably similarly an “unknown” should be processed.

[Troon]

Nice FR, voted. To save anyone checking, it also behaves like this when called as a function:

[Ildar_Gabdullin]

Nick, no, please check this:

[nickrout]

Sorry realised that and deleted post. Sorry for the noise.