Fix folderManager NPE issue in WALNode when starting iotdb in a disk-full state

[OneSizeFitsQuorum]

This PR addresses an NPE issue in WALNode creation that occurs when IoTDB starts with a full disk. When the disk is full at startup, the FolderManager initialization in the constructor fails, leaving it null. If disk space is later freed, subsequent WAL node creation attempts fail with NPE because the folderManager is still null.