Null check tracker in Entity#resendPossiblyDesyncedEntityData

Accidentally dropped this check last commit. It is possible
that this is called while the entity is not tracked.

patches/server/Properly-resend-entities.patch

@@ -138,6 +138,9 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +        if (player.getBukkitEntity().canSee(this.getBukkitEntity())) {
 +            ServerLevel world = (net.minecraft.server.level.ServerLevel)this.level();
 +            net.minecraft.server.level.ChunkMap.TrackedEntity tracker = world == null ? null : world.getChunkSource().chunkMap.entityMap.get(this.getId());
++            if (tracker == null) {
++                return;
++            }
 +            final net.minecraft.server.level.ServerEntity serverEntity = tracker.serverEntity;
 +            final List<net.minecraft.network.protocol.Packet<? super net.minecraft.network.protocol.game.ClientGamePacketListener>> list = new java.util.ArrayList<>();
 +            serverEntity.sendPairingData(player, list::add);