Support tagged not null marks

Author: EdwarDDay

core/commonMain/src/kotlinx/serialization/internal/Tagged.kt

@@ -30,6 +30,7 @@ public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
     protected open fun encodeTaggedValue(tag: Tag, value: Any): Unit =
         throw SerializationException("Non-serializable ${value::class} is not supported by ${this::class} encoder")
 
+    protected open fun encodeTaggedNonNullMark(tag: Tag) {}
     protected open fun encodeTaggedNull(tag: Tag): Unit = throw SerializationException("null is not supported")
     protected open fun encodeTaggedInt(tag: Tag, value: Int): Unit = encodeTaggedValue(tag, value)
     protected open fun encodeTaggedByte(tag: Tag, value: Byte): Unit = encodeTaggedValue(tag, value)
@@ -61,7 +62,7 @@ public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
         return true
     }
 
-    final override fun encodeNotNullMark() {} // Does nothing, open because is not really required
+    open override fun encodeNotNullMark(): Unit = encodeTaggedNonNullMark(currentTag)
     open override fun encodeNull(): Unit = encodeTaggedNull(popTag())
     final override fun encodeBoolean(value: Boolean): Unit = encodeTaggedBoolean(popTag(), value)
     final override fun encodeByte(value: Byte): Unit = encodeTaggedByte(popTag(), value)

formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonEncoder.kt

@@ -46,6 +46,7 @@ private sealed class AbstractJsonTreeEncoder(
     abstract fun putElement(key: String, element: JsonElement)
     abstract fun getCurrent(): JsonElement
 
+    override fun encodeNotNullMark() {}
 
     override fun encodeNull() {
         val tag = currentTagOrNull ?: return nodeConsumer(JsonNull)