Fix nullability of LocalCertificateSelectionCallback return

src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/MsQuicConfiguration.cs

@@ -39,21 +39,25 @@ public static MsQuicConfigurationSafeHandle Create(QuicClientConnectionOptions o
         }
         else if (authenticationOptions.LocalCertificateSelectionCallback != null)
         {
-            X509Certificate selectedCertificate = authenticationOptions.LocalCertificateSelectionCallback(
+            X509Certificate? selectedCertificate = authenticationOptions.LocalCertificateSelectionCallback(
                 options,
                 authenticationOptions.TargetHost ?? string.Empty,
                 authenticationOptions.ClientCertificates ?? new X509CertificateCollection(),
                 null,
                 Array.Empty<string>());
-            if (selectedCertificate.HasPrivateKey())
-            {
-                certificate = selectedCertificate;
-            }
-            else
+
+            if (selectedCertificate is not null)
             {
-                if (NetEventSource.Log.IsEnabled())
+                if (selectedCertificate.HasPrivateKey())
                 {
-                    NetEventSource.Info(options, $"'{certificate}' not selected because it doesn't have a private key.");
+                    certificate = selectedCertificate;
+                }
+                else
+                {
+                    if (NetEventSource.Log.IsEnabled())
+                    {
+                        NetEventSource.Info(options, $"'{certificate}' not selected because it doesn't have a private key.");
+                    }
                 }
             }
         }

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

@@ -36,7 +36,7 @@ public enum EncryptionPolicy
         [System.ObsoleteAttribute("EncryptionPolicy.NoEncryption and AllowEncryption significantly reduce security and should not be used in production code.", DiagnosticId = "SYSLIB0040", UrlFormat = "https://aka.ms/dotnet-warnings/{0}")]
         NoEncryption = 2,
     }
-    public delegate System.Security.Cryptography.X509Certificates.X509Certificate LocalCertificateSelectionCallback(object sender, string targetHost, System.Security.Cryptography.X509Certificates.X509CertificateCollection localCertificates, System.Security.Cryptography.X509Certificates.X509Certificate? remoteCertificate, string[] acceptableIssuers);
+    public delegate System.Security.Cryptography.X509Certificates.X509Certificate? LocalCertificateSelectionCallback(object sender, string targetHost, System.Security.Cryptography.X509Certificates.X509CertificateCollection localCertificates, System.Security.Cryptography.X509Certificates.X509Certificate? remoteCertificate, string[] acceptableIssuers);
     public sealed partial class NegotiateAuthentication : System.IDisposable
     {
         public NegotiateAuthentication(System.Net.Security.NegotiateAuthenticationClientOptions clientOptions) { }

src/libraries/System.Net.Security/src/System/Net/Security/SslStream.Protocol.cs

@@ -686,7 +686,7 @@ private bool AcquireServerCredentials(ref byte[]? thumbPrint)
                 if (localCertificate == null)
                 {
                     if (NetEventSource.Log.IsEnabled())
-                        NetEventSource.Error(this, $"ServerCertSelectionDelegate returned no certificaete for '{_sslAuthenticationOptions.TargetHost}'.");
+                        NetEventSource.Error(this, $"ServerCertSelectionDelegate returned no certificate for '{_sslAuthenticationOptions.TargetHost}'.");
                     throw new AuthenticationException(SR.net_ssl_io_no_server_cert);
                 }
 

src/libraries/System.Net.Security/src/System/Net/Security/SslStream.cs

@@ -33,7 +33,7 @@ public enum EncryptionPolicy
     public delegate bool RemoteCertificateValidationCallback(object sender, X509Certificate? certificate, X509Chain? chain, SslPolicyErrors sslPolicyErrors);
 
     // A user delegate used to select local SSL certificate.
-    public delegate X509Certificate LocalCertificateSelectionCallback(object sender, string targetHost, X509CertificateCollection localCertificates, X509Certificate? remoteCertificate, string[] acceptableIssuers);
+    public delegate X509Certificate? LocalCertificateSelectionCallback(object sender, string targetHost, X509CertificateCollection localCertificates, X509Certificate? remoteCertificate, string[] acceptableIssuers);
 
     public delegate X509Certificate ServerCertificateSelectionCallback(object sender, string? hostName);