Make LoadingItem assertion an optional type

Because the placeholder can be null (e.g. when scrolling the post list
very quickly), the assertion will crash unless the asserted type is
optional. This crash will not happen in production, since we already
guard with a `?.let`. This fix prevents the crash in development.

Author: mkevins

WordPress/src/main/java/org/wordpress/android/ui/posts/adapters/PostListAdapter.kt

@@ -95,7 +95,7 @@ class PostListAdapter(
         }
         if (holder is LoadingViewHolder) {
             val item = getItem(position)
-            assert(item is LoadingItem) {
+            assert(item is LoadingItem?) {
                 "If we are presenting LoadingViewHolder, the item has to be of type LoadingItem " +
                         "for position: $position"
             }