Make it possible to return an FileActionResult with success true

[brannmark]

We have a scenario were we want to import items thru a custom import action. But if import fails it should return a list of the imported items together with a status for each item. Works great.

But if all import goes well, we just want to return an empty FileActionResult with just status and notification that everything worked.
This doesn't work today as UIBuilder API fails as it cant read the response.