[etos_flutter] Make StateBuilder more 'null-safe'

[SEGVeenstra]

Sometimes a page is only shown when the app is in a specific state.
But the StateBuilder does not know this so therefor you have to do some 'pointless' checks for example:

Let's say the ItemsPage is only shown when the user is logged in, our AppState is a Union type that can be either Authenticated or Unauthenticated. It only contains items when being Authenticated.

A StateBuilder could look like this:

StateBuilder<AppState, List<Item>>(
  //option 1:
  converter: (state) => state.tryCast<Authenticated>()?.items ?? [], // We have to provide a default value or make the type nullable
  // option 2:
  converter: (state) => state.cast<Authenticated>().items, // This throws an error when the app is not `Authenticated`.
  builder: (_, items) {
    ...
  }

[SEGVeenstra]

This might actually benefit from a buildWhen method like in flutter_bloc