ReactiveCocoa/ReactiveCocoa — pull request #1045 by jspahrsummers (2014-01-06 19:02:44+00:00)
Title: Allow nil inner streams when flattening

After #941, we started hitting the assertion pretty frequently. The usual culprit was something like this:

```objc
[signal flattenMap:^(id _) {
@strongify(self);
return [self methodReturningASignal];
}];
```

Arguably, we should be more vigilant in checking for `nil`, but it's also a bit silly that returning `nil` doesn't do something sane. This makes `nil` behave like you returned an empty stream.