Don't require an Output node to be present

@danhodge - Allow EMR activities with no output node specified.  Don't merge this until we've tested on staging.
It occurs to me that this method is unnecessary as it is only used internally to this class. I think it would be better to get rid of it and rewrite the `data_sink` method as:
```ruby
def data_sink
  @data_sink ||= (emr_node.output_path ? S3DataSink.new(S3URI.parse(emr_node.output_path)) : InMemoryDataSink.new("EMR Output"))
end
```