runLambda needs to explicitly return void

[nlslatt]

Describe the bug
The ARM Intel compiler on Gitlab CI emits this warning:

/.../vt/src/vt/runnable/runnable.h(245): warning #1011: missing return statement at end of non-void function "vt::runnable::RunnableNew::runLambda(Callable &&, Args &&...) [with Callable=bool (NodeObj::*)(), Args=<NodeObj *>]"

about this code from runnable/runnable.h:

  /**
   * \brief Run the task as a lambda!
   */
  template <typename Callable, typename... Args>
  decltype(auto) runLambda(Callable&& c, Args&&... args) {
    start();
    if constexpr(std::is_void_v<std::invoke_result_t<Callable, Args...>>) {
      std::invoke(std::forward<Callable>(c), std::forward<Args>(args)...);
      finish();
    } else {
      decltype(auto) r{std::invoke(std::forward<Callable>(c), std::forward<Args>(args)...)};
      finish();
      return r;
    }
  }

We probably just need to explicitly return; for the case where the lambda returns void.

[PhilMiller]

If adding return; doesn't fix things, we could potentially rework this to something like

  template <typename Callable, typename... Args>
  decltype(auto) runLambda(Callable&& c, Args&&... args) {
    start();

    // Arrange a scope guard to call finish() without any sort of dynamic allocation
    struct finisher {
       Runnable *r;
       ~finisher() { r->finish(); }
    };
    finisher f(this);

    return std::invoke(std::forward<Callable>(c), std::forward<Args>(args)...);
  }

[cz4rs]

The same warning is visible in nvcc output (#2104).