beacon/engine, eth/catalyst: preserve nil blob list JSON

beacon/engine/bapl_encode.go

@@ -22,6 +22,9 @@ import (
 
 // MarshalJSON implements json.Marshaler.
 func (list BlobAndProofListV1) MarshalJSON() ([]byte, error) {
+	if list == nil {
+		return []byte("null"), nil
+	}
 	var b jsonw.Buffer
 	b.Array(func() {
 		for _, item := range list {
@@ -46,6 +49,9 @@ func marshalBlobAndProofV1(b *jsonw.Buffer, item *BlobAndProofV1) {
 
 // MarshalJSON implements json.Marshaler.
 func (list BlobAndProofListV2) MarshalJSON() ([]byte, error) {
+	if list == nil {
+		return []byte("null"), nil
+	}
 	var b jsonw.Buffer
 	b.Array(func() {
 		for _, item := range list {

beacon/engine/bapl_encode_test.go

@@ -0,0 +1,50 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package engine
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestBlobAndProofListMarshalJSONNil(t *testing.T) {
+	tests := []struct {
+		name string
+		list any
+	}{
+		{
+			name: "should encode null if BlobAndProofListV1 is nil",
+			list: BlobAndProofListV1(nil),
+		},
+		{
+			name: "should encode null if BlobAndProofListV2 is nil",
+			list: BlobAndProofListV2(nil),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			enc, err := json.Marshal(test.list)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if string(enc) != "null" {
+				t.Fatalf("got %s, want null", enc)
+			}
+		})
+	}
+}

eth/catalyst/api_test.go

@@ -22,6 +22,7 @@ import (
 	"crypto/ecdsa"
 	crand "crypto/rand"
 	"crypto/sha256"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"math/big"
@@ -2105,6 +2106,13 @@ func runGetBlobs(t testing.TB, getBlobs getBlobsFn, start, limit int, fillRandom
 		} else {
 			// Nil is expected if getBlobs can not return a partial response
 			expect = nil
+			enc, err := json.Marshal(result)
+			if err != nil {
+				t.Fatalf("Failed to encode result for case %s: %v", name, err)
+			}
+			if string(enc) != "null" {
+				t.Fatalf("Unexpected JSON result for case %s: got %s, want null", name, enc)
+			}
 		}
 	}
 	if !reflect.DeepEqual(result, expect) {