Recover null CraftInventory contents on 26.2

[slyphmp4]

Prevents Bukkit inventory consumers such as CoreProtect from crashing when a Cardboard ContainerBridge#getContents() implementation unexpectedly returns null.

• Injects a null-content recovery path into CraftInventory.asCraftMirror(...).
• Rebuilds the real inventory contents from the backing NMS Container slots instead of pretending the inventory is empty.
• Handles a malformed null NMS item defensively as an empty Bukkit slot.
• Logs the offending concrete container class only once, so the underlying bridge implementation can be identified without log spam.

Runtime-tested successfully together with #22. #22 has now been merged into ver/26.2; this PR contains only the CraftInventory recovery fix.