message: handle empty case
diff: @@ -114,7 +114,7 @@ let addon = {
  return;
  }
  - return this._super.call(this, new Funnel(this._broccoliConnector().tree, { srcDir: 'app' }));
  + return this._super.call(this, new Funnel(this._broccoliConnector().tree, { srcDir: 'app', allowEmpty: true }));
  },
  treeForAddon() {
  @@ -123,7 +123,7 @@ let addon = {
  return;
  }
  - let tree = new Funnel(this._broccoliConnector().tree, { srcDir: 'addon' });
  + let tree = new Funnel(this._broccoliConnector().tree, { srcDir: 'addon', allowEmpty: true });
  return this.preprocessJs(tree, '/', this.name, {
  registry: this.registry,
  });
  