Fix unit tests

Tests/MQTTnet.Core.Tests/MqttPacketReaderTests.cs

@@ -0,0 +1,20 @@
+using System.IO;
+using System.Threading;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Core.Serializer;
+
+namespace MQTTnet.Core.Tests
+{
+    [TestClass]
+    public class MqttPacketReaderTests
+    {
+        [TestMethod]
+        public void MqttPacketReader_EmptyStream()
+        {
+            var memStream = new MemoryStream();
+            var header = MqttPacketReader.ReadHeaderFromSource(memStream, CancellationToken.None);
+
+            Assert.IsNull(header);
+        }
+    }
+}