When using Dagger Multibinding, you can use methods like the below to make sure that there's a (potentially empty) Set binding for your type:

@Provides @ElementsIntoSet Set<MyType> provideEmptySetOfMyType() {
  return new HashSet<>();
}

However, there's a slightly easier way to express this:

@Multibinds abstract Set<MyType> provideEmptySetOfMyType();