Investigate the CheckpointManager::checkpointList to understand if the list is never empty by definition

At first look:
1) the first checkpoint is created in CheckpointManager::CheckpointManager
2) checkpoints can be removed only when they are closed and unreferenced
3) the open checkpoint is closed only when a new open one is created

All the above implies that the CheckpointManager::checkpointList can never be empty.
We need to verify that, as it would allow some code refactor/simplification.