fix null reference warnings in Benchmarks

seems like the warnings were (somehow) disabled here
before and were fixed by the previous TargetFrameworkIdentifier
change.

Author: mus65

test/Renci.SshNet.Benchmarks/Common/HostKeyEventArgsBenchmarks.cs

@@ -19,7 +19,7 @@ private static KeyHostAlgorithm GetKeyHostAlgorithm()
         {
             using (var s = typeof(HostKeyEventArgsBenchmarks).Assembly.GetManifestResourceStream("Renci.SshNet.Benchmarks.Data.Key.RSA.txt"))
             {
-                var privateKey = new PrivateKeyFile(s);
+                var privateKey = new PrivateKeyFile(s!);
                 return (KeyHostAlgorithm)privateKey.HostKeyAlgorithms.First();
             }
         }

test/Renci.SshNet.Benchmarks/Security/Cryptography/ED25519DigitalSignatureBenchmarks.cs

@@ -21,7 +21,7 @@ public ED25519DigitalSignatureBenchmarks()
 
             using (var s = typeof(ED25519DigitalSignatureBenchmarks).Assembly.GetManifestResourceStream("Renci.SshNet.Benchmarks.Data.Key.OPENSSH.ED25519.txt"))
             {
-                _key = (ED25519Key)new PrivateKeyFile(s).Key;
+                _key = (ED25519Key)new PrivateKeyFile(s!).Key;
             }
             _signature = new ED25519DigitalSignature(_key).Sign(_data);
         }

test/Renci.SshNet.Benchmarks/Security/Cryptography/RsaDigitalSignatureBenchmarks.cs

@@ -23,7 +23,7 @@ public RsaDigitalSignatureBenchmarks()
 
             using (var s = typeof(RsaDigitalSignatureBenchmarks).Assembly.GetManifestResourceStream("Renci.SshNet.Benchmarks.Data.Key.OPENSSH.RSA.txt"))
             {
-                _key = (RsaKey)new PrivateKeyFile(s).Key;
+                _key = (RsaKey)new PrivateKeyFile(s!).Key;
             }
             _signature = new RsaDigitalSignature(_key, HashAlgorithmName.SHA256).Sign(_data);
         }