Null value for number_missing_values

[ArlindKadra]

When using the data features, while passing through each feature, I found a null value for number_missing_values. The following code can replicate the problem:

OpenmlConnector connector = new OpenmlConnector("https://www.openml.org/", "9ed41f60b87fbe17054397936b96212d");
        Settings.CACHE_ALLOWED = false;
        DataFeature dataFeatures = connector.dataFeatures(2);
        for(Feature feature : dataFeatures.getFeatures()) {
            if(feature.getNumberOfMissingValues() != null && feature.getNumberOfMissingValues() instanceof Integer) {
                continue;
            } else {
                throw new IllegalArgumentException();
            }
        }