fix(app): allow empty sub-module invokation arguments

Previously, when instantiating a sub-module after providing global
configuration details, we were still requiring an `options` object, even
though it is meant to be optional.

lib/datastore/index.js

@@ -114,6 +114,7 @@ Datastore.dataset = Dataset;
  * @return {module:datastore/dataset}
  */
 Datastore.prototype.dataset = function(options) {
+  options = options || {};
   // Mix in global config data to the provided options.
   return new Dataset(util.extendGlobalConfig(this.config, options));
 };

lib/storage/index.js

@@ -157,6 +157,7 @@ Storage.bucket = Bucket;
  * });
  */
 Storage.prototype.bucket = function(options) {
+  options = options || {};
   // Mix in instance config data to the provided options.
   return new Bucket(util.extendGlobalConfig(this.config, options));
 };