Fix NPE in CarryOnDataSyncHandler causing server-wide disconnects on player death

[Mheaus]

Problem

When any player dies, NeoForge triggers an attachment sync for all players to all observers. sendToPlayer() already guards against syncing data from dead or just-spawned players, but not to players in those states.

When the recipient (to) is dying or respawning, the RegistryFriendlyByteBuf internals have a null ObjectArrayList (wrapped == null), causing:

Failed encoding custom payload carryon:sync_carry_data:
NullPointerException: Cannot invoke "ObjectArrayList.get(int)"
because "this.wrapped" is null

This crashes the packet encoder and kicks every connected player simultaneously. Reliably reproducible: any player death while others are online. Reported in #945, #961, #972.

Fix

Apply the same liveness guards to the recipient (to) that already exist for the holder (player) — one extra if block, mirroring the existing logic and its comments:

if (!to.isAlive() || to.isRemoved() || to.tickCount <= 0)
    return false;

Testing

Reproduced on a NeoForge 21.1.234 server with multiple players online. Before the fix: any player death causes a server-wide disconnect. The fix has not been tested yet, I generated the pr with claude code, I hope that this can help in any way,

Feel free to leave a comment or review to let me know if it's ok or not.

Note: the 1.21.1 branch already addresses this differently by commenting out the sync handler entirely. This PR brings an equivalent fix to 1.21.11 (and by extension 1.21.8/1.21.9) where the sync handler is still active.