If value transformers return `nil` for legit reasons, `-mtl_transformedValue:` right now incorrectly assumes that to be an error case.

The [NSValueTransformer Class reference](https://developer.apple.com/library/ios/documentation/cocoa/reference/foundation/Classes/NSValueTransformer_Class/Reference/Reference.html) doesn't say that `nil` is not a valid return value, guess we can't make that the default behavior, then.