Allow return null from ConnectionListener.AcceptAsync (#41453)

Co-authored-by: Natalia Kondratyeva <knatalia@microsoft.com>

Backport of #41442 to release/5.0

As it was discussed in #41304, with current System.Net.Connections APIs, listeners need to throw from AcceptAsync when shutting down since AcceptAsync cannot return null. This PR allows AcceptAsync to return null, to avoid unnecessary first-chance exceptions when listeners are gracefully shutting down.

Author: github-actions[bot]

src/libraries/System.Net.Connections/ref/System.Net.Connections.cs

@@ -52,7 +52,7 @@ public abstract partial class ConnectionListener : System.IAsyncDisposable, Syst
         protected ConnectionListener() { }
         public abstract System.Net.Connections.IConnectionProperties ListenerProperties { get; }
         public abstract System.Net.EndPoint? LocalEndPoint { get; }
-        public abstract System.Threading.Tasks.ValueTask<System.Net.Connections.Connection> AcceptAsync(System.Net.Connections.IConnectionProperties? options = null, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken));
+        public abstract System.Threading.Tasks.ValueTask<System.Net.Connections.Connection?> AcceptAsync(System.Net.Connections.IConnectionProperties? options = null, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken));
         public void Dispose() { }
         protected virtual void Dispose(bool disposing) { }
         public System.Threading.Tasks.ValueTask DisposeAsync() { throw null; }

src/libraries/System.Net.Connections/src/System/Net/Connections/ConnectionListener.cs

@@ -27,7 +27,7 @@ public abstract class ConnectionListener : IAsyncDisposable, IDisposable
         /// <param name="options">Options used to create the connection, if any.</param>
         /// <param name="cancellationToken">A token used to cancel the asynchronous operation.</param>
         /// <returns>A <see cref="ValueTask{TResult}"/> for the <see cref="Connection"/>.</returns>
-        public abstract ValueTask<Connection> AcceptAsync(IConnectionProperties? options = null, CancellationToken cancellationToken = default);
+        public abstract ValueTask<Connection?> AcceptAsync(IConnectionProperties? options = null, CancellationToken cancellationToken = default);
 
         public void Dispose()
         {