Fix nullability warnings and annotations

Author: drewnoakes

src/NetMQ.Tests/NetMQCertificateTest.cs

@@ -13,7 +13,7 @@ public void X85()
             for (int i = 0; i < 1000; i++)
             {
                 var key = new NetMQCertificate();
-                var copy  = new NetMQCertificate(key.SecretKeyZ85, key.PublicKeyZ85);
+                var copy = new NetMQCertificate(key.SecretKeyZ85!, key.PublicKeyZ85);
 
                 Assert.Equal(key.SecretKeyZ85, copy.SecretKeyZ85);
                 Assert.Equal(key.PublicKeyZ85, copy.PublicKeyZ85);
@@ -40,7 +40,7 @@ public void FromPublicKey()
         public void FromSecretKey()
         {
             var key = new NetMQCertificate();
-            var copy = new NetMQCertificate().FromSecretKey(key.SecretKeyZ85);
+            var copy = new NetMQCertificate().FromSecretKey(key.SecretKeyZ85!);
 
             Assert.Equal(key.SecretKeyZ85, copy.SecretKeyZ85);
             Assert.Equal(key.PublicKeyZ85, copy.PublicKeyZ85);

src/NetMQ/NetMQActor.cs

@@ -149,8 +149,8 @@ private NetMQActor(PairSocket self, PairSocket shim, IShimHandler shimHandler)
             m_self = self;
             m_shim = shim;
 
-            void OnReceive(object sender, NetMQSocketEventArgs e) => m_receiveEvent.Fire(this, new NetMQActorEventArgs(this));
-            void OnSend   (object sender, NetMQSocketEventArgs e) => m_sendEvent   .Fire(this, new NetMQActorEventArgs(this));
+            void OnReceive(object sender, NetMQSocketEventArgs e) => m_receiveEvent!.Fire(this, new NetMQActorEventArgs(this));
+            void OnSend   (object sender, NetMQSocketEventArgs e) => m_sendEvent   !.Fire(this, new NetMQActorEventArgs(this));
 
             m_receiveEvent = new EventDelegator<NetMQActorEventArgs>(
                 () => m_self.ReceiveReady += OnReceive,

src/NetMQ/NetMQBeacon.cs

@@ -1,10 +1,11 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Net;
 using System.Net.Sockets;
+using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
 using NetMQ.Sockets;
-using System.Runtime.InteropServices;
 
 namespace NetMQ
 {
@@ -199,7 +200,7 @@ private void OnUdpReady(Socket socket)
             {
                 Assumes.NotNull(m_pipe);
 
-                if (!TryReceiveUdpFrame(out NetMQFrame frame, out string peerName))
+                if (!TryReceiveUdpFrame(out NetMQFrame? frame, out string? peerName))
 		    return;
 
                 // If filter is set, check that beacon matches it
@@ -279,7 +280,7 @@ private void SendUdpFrame(NetMQFrame frame)
                 }
             }
 
-            private bool TryReceiveUdpFrame(out NetMQFrame frame, out string peerName)
+            private bool TryReceiveUdpFrame([NotNullWhen(returnValue: true)] out NetMQFrame? frame, [NotNullWhen(returnValue: true)] out string? peerName)
             {
                 Assumes.NotNull(m_udpSocket);
 
@@ -321,7 +322,7 @@ public NetMQBeacon()
         {
             m_actor = NetMQActor.Create(new Shim());
 
-            void OnReceive(object sender, NetMQActorEventArgs e) => m_receiveEvent.Fire(this, new NetMQBeaconEventArgs(this));
+            void OnReceive(object sender, NetMQActorEventArgs e) => m_receiveEvent!.Fire(this, new NetMQBeaconEventArgs(this));
 
             m_receiveEvent = new EventDelegator<NetMQBeaconEventArgs>(
                 () => m_actor.ReceiveReady += OnReceive,

src/NetMQ/NetMQCertificate.cs

@@ -36,7 +36,7 @@ private string Z85Encode(byte[] data)
         {
             byte byte_nbr = 0;
             UInt32 value = 0;
-            string dest = null;
+            string? dest = null;
             while (byte_nbr<data.Length) 
             {
                 //  Accumulate value in base 256 (binary)
@@ -53,7 +53,7 @@ private string Z85Encode(byte[] data)
                     value = 0;
                 }
             }
-            return dest;
+            return dest ?? "";
         }
 
         /// <summary>
@@ -255,7 +255,7 @@ public static NetMQCertificate FromPublicKey(string publicKey)
         /// <summary>
         /// Curve Public key 
         /// </summary>
-        public string SecretKeyZ85 => SecretKey != null ? Z85Encode(SecretKey) : null;
+        public string? SecretKeyZ85 => SecretKey != null ? Z85Encode(SecretKey) : null;
 
 
         /// <summary>