Added check for datetime year 1 to be deserialized as null

CryptoExchange.Net/Converters/SystemTextJson/DateTimeConverter.cs

@@ -57,6 +57,7 @@ public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerial
                     var stringValue = reader.GetString();
                     if (string.IsNullOrWhiteSpace(stringValue)
                         || stringValue == "-1"
+                        || stringValue == "0001-01-01T00:00:00Z"
                         || double.TryParse(stringValue, out var doubleVal) && doubleVal == 0)
                     {
                         return default;