Strange type empty Flow syntax inside default statement of Redux reducer

I found the code sample while searching for ways to use flow with redux here: https://flow.org/en/docs/frameworks/redux/

Peculiar syntax is (action: empty); Is it just a bit of flow magic intended to be used just inside default case of switch statement or does it have other uses?

It looks like out of place function type statement without return value type but with parameter of strange type 'empty', which I couldn't find documentation about.

// @flow
type State = { +value: boolean };

type FooAction = { type: "FOO", foo: boolean };
type BarAction = { type: "BAR", bar: boolean };

type Action = FooAction | BarAction;

function reducer(state: State, action: Action): State {
  switch (action.type) {
    case "FOO": return { ...state, value: action.foo };
    case "BAR": return { ...state, value: action.bar };
    default:
      (action: empty);
      return state;
  }
}