added encode_nbt, added 'nbt' and 'nbt_merge' to modify entity API.

src/main/java/carpet/script/value/NullValue.java

@@ -1,5 +1,8 @@
 package carpet.script.value;
 
+import net.minecraft.nbt.StringTag;
+import net.minecraft.nbt.Tag;
+
 public class NullValue extends NumericValue // TODO check nonsingleton code
 {
     @Override
@@ -50,4 +53,11 @@ public class NullValue extends NumericValue // TODO check nonsingleton code
     {
         return 0;
     }
+
+    @Override
+    public Tag toTag(boolean force)
+    {
+        if (!force) throw new NBTSerializableValue.IncompatibleTypeException(this);
+        return StringTag.of("null");
+    }
 }