Message object is now optional when constructing MigrateExecutable

Previously, the \Drupal\migrate\MigrateExecutable class required an object implementing \Drupal\migrate\MigrateMessageInterface to be passed into its constructor:

use Drupal\migrate\MigrateMessage;

new MigrateExecutable($migration, new MigrateMessage());

The message object is no longer required. If it is not passed in, MigrateExecutable will create a new MigrateMessage object. So unless you need some special-sauce implementation of MigrateMessageInterface, you can just do this:

new MigrateExecutable($migration);