System.Text.Json source generator emits "" for null byte[] properties

[lezzi]

Description

The System.Text.Json source generator serializes a null byte[] property as an empty string ("") instead of null, diverging from the reflection-based serializer. The reflection path (ByteArrayConverter) writes null for a null byte[]; the generated fast path writes "", breaking round-tripping and backward compatibility.

Reproduction Steps

using System.Text.Json;
using System.Text.Json.Serialization;

var poco = new Poco(); // Data is null
Console.WriteLine(JsonSerializer.Serialize(poco));                  // reflection
Console.WriteLine(JsonSerializer.Serialize(poco, Ctx.Default.Poco)); // source-gen

class Poco { public byte[]? Data { get; set; } }

[JsonSerializable(typeof(Poco))]
partial class Ctx : JsonSerializerContext { }

Expected behavior

Both serializers produce the same output: {"Data":null}

Actual behavior

The source generator produces: {"Data":""}

Regression?

No

Known Workarounds

Use a custom converter.

Configuration

• Affects all current versions where the STJ source generator emits the Base64 fast path (reproduces on .NET 8/9/10/current main).
• Not OS- or architecture-specific.

Other information

No response

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[lezzi]

I drafted #129834, but open to feedback about best possible approach here before I officially open it.