Add `impl Modifier for ()`

[Nemo157]

This allows for null modifiers which can be useful when you may want to decide whether to return a modifier or not at either compile or run time. Runtime deciding will also need PR #20 to allow returning different types boxed, compile time deciding through cfg attributes works now.

[reem]

This needs a rebase

[Nemo157]

Rebased (finally) 😄