Reduce size of generated code when using CodecMakerConfig.withEncodingOnly(true)

Author: plokhotnyuk

jsoniter-scala-macros/shared/src/main/scala-2/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMaker.scala

@@ -2307,7 +2307,10 @@ object JsonCodecMaker {
               @_root_.java.lang.SuppressWarnings(_root_.scala.Array("org.wartremover.warts.All"))
               val x = new _root_.com.github.plokhotnyuk.jsoniter_scala.core.JsonValueCodec[$rootTpe] {
                 @_root_.scala.inline
-                def nullValue: $rootTpe = ${genNullValue(rootTpe :: Nil)}
+                def nullValue: $rootTpe = ${
+                  if (cfg.encodingOnly) q"_root_.scala.Predef.???"
+                  else genNullValue(rootTpe :: Nil)
+                }
 
                 @_root_.scala.inline
                 def decodeValue(in: _root_.com.github.plokhotnyuk.jsoniter_scala.core.JsonReader, default: $rootTpe): $rootTpe = ${

jsoniter-scala-macros/shared/src/main/scala-3/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMaker.scala

@@ -3111,7 +3111,10 @@ object JsonCodecMaker {
       val codecDef = '{ // FIXME: generate a type class instance using `ClassDef.apply` and `Symbol.newClass` calls after graduating from experimental API: https://www.scala-lang.org/blog/2022/06/21/scala-3.1.3-released.html
         new JsonValueCodec[A] {
           @inline
-          def nullValue: A = ${genNullValue[A](List(rootTpe))}
+          def nullValue: A = ${
+            if (cfg.encodingOnly) '{ ??? }
+            else genNullValue[A](List(rootTpe))
+          }
 
           @inline
           def decodeValue(in: JsonReader, default: A): A = ${