Assert nil diff identifier, remove if nil

Summary:
Do not allow `nil` diff identifiers to get through. Pluck the objects if this happens and assert.

- Fix the bug if it happens
- Assert to tell the developer what is wrong

Differential Revision: D4753827

fbshipit-source-id: 3a92543c648ed7157ec30af7cb1c91a6e6238e33

Source/IGListAdapterUpdater.m

@@ -97,7 +97,8 @@ - (void)performReloadDataWithCollectionView:(UICollectionView *)collectionView {
     NSMutableArray *uniqueObjects = [NSMutableArray new];
     for (id<IGListDiffable> object in objects) {
         id diffIdentifier = [object diffIdentifier];
-        if (![identifiers containsObject:diffIdentifier]) {
+        if (diffIdentifier != nil
+            && ![identifiers containsObject:diffIdentifier]) {
             [identifiers addObject:diffIdentifier];
             [uniqueObjects addObject:object];
         } else {
@@ -385,6 +386,8 @@ - (void)performUpdateWithCollectionView:(UICollectionView *)collectionView
     for (id obj in toObjects) {
         IGAssert([obj conformsToProtocol:@protocol(IGListDiffable)],
                  @"In order to use IGListAdapterUpdater, object %@ must conform to IGListDiffable", obj);
+        IGAssert([obj diffIdentifier] != nil,
+                 @"Cannot have a nil diffIdentifier for object %@", obj);
     }
 #endif