IGListAdapter: Fix not returning early when collectionView/dataSource is nil and completion is nil

Summary:
- I ran the static analyzer, and it found a case in `performUpdatesAnimated:completion:` where it looks like we meant to bail out early if the `collectionView` or `dataSource` is nil, but it only does so if `completion` is provided.
- Fixed a spelling error in nearby documentation

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/CONTRIBUTING.md)

- There is one other analyzer warning, but it seems to be making an incorrect assumption. In any case, it's not something I can fix quickly (requires more time spent staring at IGListDiff.mm).
- A ticket should be opened to add static analysis to .travis.yml. I'lll do this soonish, unless someone beats me to it!

- I could add a test for this, but this project is annotated well enough with null
Closes https://github.com/Instagram/IGListKit/pull/51

Reviewed By: ocrickard

Differential Revision: D4008605

Pulled By: rnystrom

fbshipit-source-id: c2b7516cb8c1fc619aa63d1e9b0c26f2918cd8a1

Source/IGListAdapter.h

@@ -103,14 +103,14 @@ IGLK_SUBCLASSING_RESTRICTED
 - (void)performUpdatesAnimated:(BOOL)animated completion:(nullable IGListUpdaterCompletion)completion;
 
 /**
- Perform an immediate reload of the data in the data source, discarding the old objectss.
+ Perform an immediate reload of the data in the data source, discarding the old objects.
 
  @param completion A block executed when the reload completes.
  */
 - (void)reloadDataWithCompletion:(nullable IGListUpdaterCompletion)completion;
 
 /**
- Reload the infra for specific objectss only.
+ Reload the infra for specific objects only.
 
  @param objects The objects to reload.
  */

Source/IGListAdapter.m

@@ -248,8 +248,8 @@
     if (dataSource == nil || collectionView == nil) {
         if (completion) {
             completion(NO);
-            return;
         }
+        return;
     }
 
     NSArray *newItems = [[dataSource objectsForListAdapter:self] copy];