Added test: should not write undefined to stream or end stream if no args are passed

This test will check if undefined is written to the stream and if the stream is ended before the actual data gets a chance to be written.

The test case may be contrived or wrong, but I think it represents what's going on between gulp and orchestrator with this gist: https://gist.github.com/lazd/7964385

Making the changes I suggested in https://github.com/robrich/execify/pull/3#discussion_r8352868 causes this test to pass, but other stream tests to fail (as they never end their streams). I modified tests to end their streams, which works with or without the change I suggested.

This is in response to https://github.com/robrich/execify/pull/3#issuecomment-30593343