[JSON] Fix source generator serializing null byte[] as empty string

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

@@ -1783,6 +1783,21 @@ private static void GenerateSerializeValueStatement(SourceWriter writer, TypeGen
                     {
                         writer.WriteLine($"writer.{primitiveWriterMethod}Value({valueExpr}.ToString());");
                     }
+                    else if (typeSpec.PrimitiveTypeKind is JsonPrimitiveTypeKind.ByteArray)
+                    {
+                        // byte[] is a nullable reference type; emit an explicit null check so that null values
+                        // serialize as 'null' rather than an empty Base64 string, matching reflection behavior.
+                        writer.WriteLine($$"""
+                            if ({{valueExpr}} is null)
+                            {
+                                writer.WriteNullValue();
+                            }
+                            else
+                            {
+                                writer.{{primitiveWriterMethod}}Value({{valueExpr}});
+                            }
+                            """);
+                    }
                     else
                     {
                         writer.WriteLine($"writer.{primitiveWriterMethod}Value({valueExpr});");
@@ -1809,6 +1824,21 @@ private static void GenerateSerializePropertyStatement(SourceWriter writer, Type
                     {
                         writer.WriteLine($"writer.{primitiveWriterMethod}({propertyNameExpr}, {valueExpr}.ToString());");
                     }
+                    else if (typeSpec.PrimitiveTypeKind is JsonPrimitiveTypeKind.ByteArray)
+                    {
+                        // byte[] is a nullable reference type; emit an explicit null check so that null values
+                        // serialize as 'null' rather than an empty Base64 string, matching reflection behavior.
+                        writer.WriteLine($$"""
+                            if ({{valueExpr}} is null)
+                            {
+                                writer.WriteNull({{propertyNameExpr}});
+                            }
+                            else
+                            {
+                                writer.{{primitiveWriterMethod}}({{propertyNameExpr}}, {{valueExpr}});
+                            }
+                            """);
+                    }
                     else
                     {
                         writer.WriteLine($"writer.{primitiveWriterMethod}({propertyNameExpr}, {valueExpr});");

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/RealWorldContextTests.cs

@@ -873,6 +873,7 @@ public virtual void ClassWithNullableProperties_Roundtrip()
             {
                 Uri = new Uri("http://contoso.com"),
                 Array = new int[] { 42 },
+                ByteArray = new byte[] { 1, 2, 3 },
                 Poco = new ClassWithNullableProperties.MyPoco(),
 
                 NullableUri = new Uri("http://contoso.com"),
@@ -889,6 +890,7 @@ void RunTest(ClassWithNullableProperties expected)
 
                 Assert.Equal(expected.Uri, actual.Uri);
                 Assert.Equal(expected.Array, actual.Array);
+                Assert.Equal(expected.ByteArray, actual.ByteArray);
                 Assert.Equal(expected.Poco, actual.Poco);
 
                 Assert.Equal(expected.NullableUri, actual.NullableUri);
@@ -943,6 +945,7 @@ public class ClassWithNullableProperties
         {
             public Uri? Uri { get; set; }
             public int[]? Array { get; set; }
+            public byte[]? ByteArray { get; set; }
             public MyPoco? Poco { get; set; }
 
             public Uri? NullableUri { get; set; }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/SerializationContextTests.cs

@@ -455,6 +455,7 @@ public override void ClassWithNullableProperties_Roundtrip()
             {
                 Uri = new Uri("http://contoso.com"),
                 Array = new int[] { 42 },
+                ByteArray = new byte[] { 1, 2, 3 },
                 Poco = new ClassWithNullableProperties.MyPoco(),
 
                 NullableUri = new Uri("http://contoso.com"),
@@ -471,6 +472,7 @@ void RunTest(ClassWithNullableProperties expected)
 
                 Assert.Equal(expected.Uri, actual.Uri);
                 Assert.Equal(expected.Array, actual.Array);
+                Assert.Equal(expected.ByteArray, actual.ByteArray);
                 Assert.Equal(expected.Poco, actual.Poco);
 
                 Assert.Equal(expected.NullableUri, actual.NullableUri);