Fix AsyncShell

Fix issue with initial Promise in the
CancellableInputProvider.tuple not being completed,
which would violate the constraint in
CancellableInputProvider.nextAction.

src/main/scala/com/nthportal/shell/async/AsyncShell.scala

@@ -77,7 +77,7 @@ object AsyncShell {
     */
   private class CancellableInputProvider(inputProvider: InputProvider) extends InputProvider {
     private val tuple: AtomicReference[(Boolean, Promise[InputAction[_]])] =
-      new AtomicReference((false, Promise[InputAction[_]]()))
+      new AtomicReference((false, Promise.successful[InputAction[_]](_ => Unit)))
 
     /**
       * Attempts to cancel/halt this [[InputProvider]].