Fix crash when lectern has no item

This should never happen in practice, but might happen when using
/setblock or after world corruption, so let's be careful here.

Closes #2014

projects/common/src/main/java/dan200/computercraft/shared/lectern/CustomLecternBlockEntity.java

@@ -119,7 +119,7 @@ public Packet<ClientGamePacketListener> getUpdatePacket() {
     @Override
     public CompoundTag getUpdateTag() {
         var tag = super.getUpdateTag();
-        tag.put(NBT_ITEM, item.save(new CompoundTag()));
+        if (!item.isEmpty()) tag.put(NBT_ITEM, item.save(new CompoundTag()));
         return tag;
     }