reproduce nullable boxing issue; to be run using the benchmark added in

dotnet/performance#1920

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/NullableConverter.cs

@@ -70,7 +70,5 @@ internal override void WriteNumberWithCustomHandling(Utf8JsonWriter writer, T? v
                 _converter.WriteNumberWithCustomHandling(writer, value.Value, handling);
             }
         }
-
-        internal override bool IsNull(in T? value) => !value.HasValue;
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs

@@ -312,7 +312,7 @@ internal override sealed bool TryReadAsObject(ref Utf8JsonReader reader, JsonSer
         /// <summary>
         /// Overridden by the nullable converter to prevent boxing of values by the JIT.
         /// </summary>
-        internal virtual bool IsNull(in T value) => value == null;
+        private bool IsNull(in T value) => value == null;
 
         internal bool TryWrite(Utf8JsonWriter writer, in T value, JsonSerializerOptions options, ref WriteStack state)
         {