仅在明确需要的场景下允许item为空, fix #3793 #3805
                
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避免为了SingleItemAdapter特例做出过多影响使用体验的让步
允许重载 getItem;
仅在明确需要的场景下允许item为空, fix 4.0.0版本onBindViewHolder(@NonNull QuickViewHolder holder, int viewType, @Nullable T item)可空修改 #3793;
提供默认item为空的 SimpleSingleItemAdapter
-为SimpleSingleItemAdapter 提供默认为空的 onBindViewHolde