Change back to Require.NotNull,

`Require.NotNull` should be replaced with `ArgumentNullException.ThrowIfNull`

Author: xPaw

ValveKeyValue/ValveKeyValue/ObjectCopier.cs

@@ -149,12 +149,7 @@ static KVValue ConvertObjectToValue(
         static void CopyObject(KVObject kv, [DynamicallyAccessedMembers(Trimming.Properties)] Type objectType, object obj, IObjectReflector reflector)
         {
             Require.NotNull(kv, nameof(kv));
-
-            if (obj is null)
-            {
-                throw new ArgumentNullException(nameof(obj));
-            }
-
+            Require.NotNull(obj, nameof(obj));
             Require.NotNull(reflector, nameof(reflector));
 
             var members = reflector.GetMembers(objectType, obj).ToDictionary(m => m.Name, m => m, StringComparer.OrdinalIgnoreCase);