fix: make Runner's concatAll safer

[AndersDJohnson]

I have noticed that in some cases the concatAll function in Runner.js results in errors. Looks like the function assumes a fully populated nested array, whereas the code that generates the nested array with which this function is called can actually result in undefined nested items by resolving the promise with nothing here:

jscodeshift/src/Runner.js

Line 142 in d63aa84

resolve();

If it is intentional to throw an error in this case rather than proceeding, I would at least prefer if such errors were handled more gracefully, as I saw no indication as to the root cause in the logs or stack trace when this occurred.

Alternatively, we could change the resolve() to resolve([]), as is done below it. It looks like this was attempted in #334.

Let me know if we should add unit tests to cover this possibility and prove this works - I just wanted to raise a minimal PR to clearly demonstrate the change in functionality to start.