IssueTracker

Component used: WorkManager
Version used: alpha09
Devices/Android versions reproduced on:

When using WorkContinuation.combine to join multiple works before continuing, it has a check in it to make sure it's acutally combining multiple WorkContinuations. I think this check is uneccessary and annoying, and adds a hard to find bug in implementations. The combine() method could easily just do nothing if there's only a single WorkContinuation passed to it.

Current use case:
I'm uploading a number of photos, which I'm splitting into a number of WorkContinuation lists so that several happen in parallel. If I only have one photo to upload then obviously I can only split this into one list of work, and the app crashes because WorkContinuation.combine() 'cannot' combine a single item. To fix this I have to have this code:

```
val allCombined = if (workLists.size == 1) workLists[0] else WorkContinuation.combine(workLists)
allCombined.then(....
```

which is much less nice than the original code which I think should work:

```
WorkContinuation.combine(workLists)
.then(....
```