I struggle to find a way writing this value transformer without exposing a new, common superclass for `MTLValueTransformer` and `MTLDeprecatedValueTransformer` or duplicating the old code.

I'm not sure the difference of a transformer lacking error handling machinery and one that treats `nil` as success is that big though, especially if this method is being deprecated. What if we left it like this for the time being and go back to it once we see how it actually interacts with the adapters?

Alternatively, we could mimick `MTLModel (Unavailable)` and remove them altogether?