Fix sending MidiPacket when MidiPacket has been created using a pointer (#18981)

Fix sending MidiPacket when MidiPacket has been created using a pointer to a byte[] rather than a byte[] passed as a reference.

MidiPacket.bytes used to be initialized to null, but that changed in a null-refactoring. Unfortunately code that dependend on the initial value being null wasn't updated, so the code regressed. Here we revert the old behavior by making the MidiPacket.bytes field nullable

Also add a test to make sure we don't regress this code again.

---------

Co-authored-by: Alex Soto <alex@alexsoto.me>
Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>

src/CoreMidi/MidiServices.cs

@@ -791,7 +791,7 @@ namespace CoreMidi {
 #if !COREBUILD
 		public long TimeStamp;
 		IntPtr byteptr;
-		byte [] bytes = Array.Empty<byte> ();
+		byte []? bytes;
 		int start;
 		public ushort Length;
 
@@ -847,7 +847,7 @@ namespace CoreMidi {
 			byteptr = IntPtr.Zero;
 		}
 
-		internal byte [] ByteArray {
+		internal byte []? ByteArray {
 			get { return bytes; }
 		}
 

tests/monotouch-test/CoreMidi/MidiClientTest.cs

@@ -0,0 +1,47 @@
+//
+// Unit tests for MidiClient
+//
+// Authors:
+//	Rolf Bjarne Kvinge <rolf@xamarin.com>
+//
+// Copyright 2023 Microsoft Corp. All rights reserved.
+//
+
+#if !__TVOS__ && !__WATCHOS__
+using System;
+using System.Diagnostics;
+
+using CoreMidi;
+using Foundation;
+
+using NUnit.Framework;
+
+namespace MonoTouchFixtures.CoreMidi {
+	[TestFixture]
+	[Preserve (AllMembers = true)]
+	public class MidiClientTest {
+		[Test]
+		public void SendTest ()
+		{
+			if (Midi.DestinationCount <= 0)
+				Assert.Inconclusive ("No Midi Destinations");
+
+			using var ep = MidiEndpoint.GetDestination (0);
+			Assert.NotNull (ep, "EndPoint");
+
+			var mevent = new byte [] { 0x90, 0x40, 0x70 };
+			using var client = new MidiClient ($"outputclient-{Process.GetCurrentProcess ().Id}");
+			using var port = client.CreateOutputPort ($"outputport-{Process.GetCurrentProcess ().Id}");
+			unsafe {
+				fixed (byte* meventPtr = mevent) {
+					using var packet1 = new MidiPacket (0, (ushort) mevent.Length, (IntPtr) meventPtr);
+					using var packet2 = new MidiPacket (0, mevent);
+					using var packet3 = new MidiPacket (0, mevent, 0, mevent.Length);
+					var packets = new MidiPacket [] { packet1, packet2, packet3 };
+					port.Send (ep, packets);
+				}
+			}
+		}
+	}
+}
+#endif