Fixed bug when no files were inputted

If no files were inputted to the stream, it would fail while closing the
concat stream.

This test case fails before the fix:

````
it('should not fail if no files were input', function(done) {
    var stream = concat('test.js');
    stream.end();
    done();
})
````

The fix was about enclosing the end of the stream in a simple `if` statement.

The error message can be seem on the issue #46.

Closes #46