Small improvements

CryptoExchange.Net/Converters/SystemTextJson/ArrayConverter.cs

@@ -81,7 +81,7 @@ namespace CryptoExchange.Net.Converters.SystemTextJson
             private static object ParseObject(ref Utf8JsonReader reader, object result, Type objectType)
             {
                 if (reader.TokenType != JsonTokenType.StartArray)
-                    throw new Exception("1");
+                    throw new Exception("Not an array");
 
                 if (!_typeAttributesCache.TryGetValue(objectType, out var attributes))
                     attributes = CacheTypeAttributes(objectType);

CryptoExchange.Net/ExtensionMethods.cs

@@ -96,6 +96,9 @@ namespace CryptoExchange.Net
             var formData = HttpUtility.ParseQueryString(string.Empty);
             foreach (var kvp in parameters)
             {
+                if (kvp.Value is null)
+                    continue;
+
                 if (kvp.Value.GetType().IsArray)
                 {
                     var array = (Array)kvp.Value;