I don't like this. It's cleaner than before, but now the `resolve` method is responsible of detecting whether the given path has been stored within this resolver and resolve the given path to a URL utilising the resolver.

I would prefer a new method `isStored` or something to decouple the two facts.

The controller then can distinguish between an already stored image and a to-be-stored one.
This will also remove the ambiguous return value of the `Resolver::resolve` method. It's always string: the URL. While the new method will take care of the previous `null` return value, question would result in a `false` of `isStored`.