Deal with res is null.

[evanvosberg]

In some cases nothing changed and res will be just null.
In this case the task will fail with "Cannot read property 'map' of null".

As the destination might be different from the source the file should be just copied.

[nicolo-ribaudo]

transformFileAsync returns null not when nothing is changed, but when the file is explicitly ignored. If it's ignored I think we should ignore it rather than copying it.

[evanvosberg]

In my opinion the babel-grunt task is a combination of copy and process by Babel.

The files src and destination is not necessarily the same, therefor the task includes copy beside processing by Babel.

The task defines files to copy and process by Babel, but the babel config (.babelrc) might exclude some from processing. But this is a Babel setting not a src > dest setting, therefor I think it should get copied.