Fix #1761: MQTTRetainedMessageManager, return task with NULL result instead of NULL (#1762)

* Return task with NULL result instead of NULL

* move Task.FromResult call out of lock

* Update ReleaseNotes.md

---------

Co-authored-by: Christian <6939810+chkr1011@users.noreply.github.com>

.github/workflows/ReleaseNotes.md

@@ -1 +1,2 @@
-* [Client] Fixed _PlatformNotSupportedException_ when using Blazor (#1755, thanks to @Nickztar).
+* [Client] Fixed _PlatformNotSupportedException_ when using Blazor (#1755, thanks to @Nickztar).
+* [Server] Fixed _NullReferenceException_ in retained messages management (#1762, thanks to @logicaloud).

Source/MQTTnet.Tests/Server/MqttRetainedMessageManager_Tests.cs

@@ -0,0 +1,25 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class MqttRetainedMessageManager_Tests
+    {
+        [TestMethod]
+        public async Task MqttRetainedMessageManager_GetUndefinedTopic()
+        {
+            var logger = new Mockups.TestLogger();
+            var eventContainer = new MQTTnet.Server.MqttServerEventContainer();
+            var retainedMessagesManager = new MQTTnet.Server.MqttRetainedMessagesManager(eventContainer, logger);
+            var task = retainedMessagesManager.GetMessage("undefined");
+            Assert.IsNotNull(task, "Task should not be null");
+            var result = await task;
+            Assert.IsNull(result, "Null result expected");
+        }
+    }
+}

Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs

@@ -134,9 +134,9 @@ namespace MQTTnet.Server
                 {
                     return Task.FromResult(message);
                 }
-
-                return null;
             }
+
+            return Task.FromResult<MqttApplicationMessage>(null);
         }
 
         public async Task ClearMessages()