Convert null checks to modern ThrowIfNull checks (#2089)

Convert null check to modern ThrowIfNull checks

Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs

@@ -25,10 +25,7 @@ public sealed class MqttV5PacketEncoder
 
     public MqttPacketBuffer Encode(MqttPacket packet)
     {
-        if (packet == null)
-        {
-            throw new ArgumentNullException(nameof(packet));
-        }
+        ArgumentNullException.ThrowIfNull(packet);
 
         // Leave enough head space for max header size (fixed + 4 variable remaining length = 5 bytes)
         const int ReservedHeaderSize = 5;

Source/MQTTnet/Formatter/V5/MqttV5PropertiesWriter.cs

@@ -221,7 +221,7 @@ namespace MQTTnet.Formatter.V5
         {
             if (value)
             {
-                // Absence of the flag means it is supported! 
+                // Absence of the flag means it is supported!
                 return;
             }
 
@@ -230,10 +230,7 @@ namespace MQTTnet.Formatter.V5
 
         public void WriteTo(MqttBufferWriter target)
         {
-            if (target == null)
-            {
-                throw new ArgumentNullException(nameof(target));
-            }
+            ArgumentNullException.ThrowIfNull(target);
 
             target.WriteVariableByteInteger((uint)_bufferWriter.Length);
             target.Write(_bufferWriter);