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();
			}
		}