Fix nullability of LocalCertificateSelectionCallback return

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);