BinaryDecoder: zero-length arrays share Array.Empty

ReadArrayElements and ReadFixedWidthArray allocated a new T[0] for a
zero-length array, which every response header has (StringTable,
DiagnosticInfos). A length of -1 still decodes as a null array.

ServerLoadHarness read --nodes 1 (20 sessions, 4 alternating rounds):
client allocB/op 1871 -> 1822 on top of the channel timer change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Opc.Ua.Types/Encoders/BinaryDecoder.cs

@@ -2432,6 +2432,12 @@ namespace Opc.Ua
         private T[] ReadArrayElements<T, TReader>(int length, TReader reader)
             where TReader : struct, IElementReader<T>
         {
+            // an empty array is immutable: all share one instance.
+            if (length == 0)
+            {
+                return Array.Empty<T>();
+            }
+
             int capacity = GetInitialArrayCapacity<T>(length);
             if (capacity < length)
             {
@@ -2526,6 +2532,11 @@ namespace Opc.Ua
         /// against the remaining bytes by the caller.</param>
         private T[] ReadFixedWidthArray<T>(int length) where T : unmanaged
         {
+            if (length == 0)
+            {
+                return Array.Empty<T>();
+            }
+
             // The caller verified the length against the remaining bytes. If
             // they are unknown the array grows with the bytes actually read.
             int capacity = GetRemainingLength() >= 0

tests/Opc.Ua.Types.Tests/Encoders/BinaryDecoderTests.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Runtime.InteropServices;
 using Moq;
 using NUnit.Framework;
 using Opc.Ua.Tests;
@@ -767,6 +768,33 @@ namespace Opc.Ua.Types.Tests.Encoders
             Assert.That(result.Count, Is.Zero);
         }
 
+        /// <summary>
+        /// Zero-length arrays, such as the empty StringTable of every response
+        /// header, decode as empty, not null, and allocate nothing.
+        /// </summary>
+        [Test]
+        public void ReadZeroLengthArraysReturnsEmptyWithoutAllocating()
+        {
+            ITelemetryContext telemetryContext = NUnitTelemetryContext.Create();
+            var messageContext = ServiceMessageContext.CreateEmpty(telemetryContext);
+            byte[] buffer = [0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF, 0xFF, 0xFF];
+            using var decoder = new BinaryDecoder(buffer, messageContext);
+
+            ArrayOf<string?> strings = decoder.ReadStringArray(null);
+            ArrayOf<int> integers = decoder.ReadInt32Array(null);
+            ArrayOf<string?> nullStrings = decoder.ReadStringArray(null);
+
+            Assert.That(strings.IsNull, Is.False);
+            Assert.That(strings.Count, Is.Zero);
+            Assert.That(MemoryMarshal.TryGetArray(strings.Memory, out ArraySegment<string?> stringArray), Is.True);
+            Assert.That(stringArray.Array, Is.SameAs(Array.Empty<string?>()));
+            Assert.That(integers.IsNull, Is.False);
+            Assert.That(integers.Count, Is.Zero);
+            Assert.That(MemoryMarshal.TryGetArray(integers.Memory, out ArraySegment<int> integerArray), Is.True);
+            Assert.That(integerArray.Array, Is.SameAs(Array.Empty<int>()));
+            Assert.That(nullStrings.IsNull, Is.True);
+        }
+
         [Test]
         public void ReadEncodeableArrayReturnsMultipleElements()
         {