Added explicit null return to completeUnitOfWork() to resolve Flow error

[bvaughn]

completeUnitOfWork causes Flow errors because of this while (true) loop:

function completeUnitOfWork(workInProgress: Fiber): Fiber | null {
  while (true) {
    // ...
  }
}

Here is a smaller repro.

Maybe we should remove or reconsider this while loop but for now I've fixed the issue by adding an explicit null return outside of the loop. (It was blocking a merge in fbsource so I fixed it there and this is an upstream sync.)

[acdlite]

Let's add a todo here so that we come back and remove while (true)

[bvaughn]

Done.