Fix exception message and add test cases.

Source/MQTTnet/Extensions/UserPropertyExtension.cs

@@ -25,7 +25,7 @@ namespace MQTTnet.Extensions
             }
             catch (Exception ex)
             {
-                throw new InvalidOperationException($"Cannot convert value({value}) of UserProperty({propertyName}) to {typeof(T).FullName}.", ex);
+                throw new InvalidOperationException($"Cannot convert value({value ?? "null"}) of UserProperty({propertyName}) to {typeof(T).FullName}.", ex);
             }
         }
     }

Tests/MQTTnet.Core.Tests/MqttApplicationMessage_Tests.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Extensions;
@@ -24,6 +25,9 @@ namespace MQTTnet.Tests
             Assert.AreEqual("bar", message.GetUserProperty("foo"));
             Assert.AreEqual(1011, message.GetUserProperty<int>("value"));
             Assert.AreEqual("insensitive", message.GetUserProperty("case"));
+            Assert.AreEqual(null, message.GetUserProperty("nonExists"));
+            Assert.AreEqual(null, message.GetUserProperty<int?>("nonExists"));
+            Assert.ThrowsException<InvalidOperationException>(() => message.GetUserProperty<int>("nonExists"));
         }
     }
 }