Field#description() should return a proper java.lang.String when documentation/description is not set

When Kafka's ConfigDef renders the HTML or RST documentation representation of the config definition, it requires documentation to be a java.lang.String instance and not being NULL, else NPE happens:

        b.append(key.documentation.replaceAll("\n", "<br>"));

        for (String docLine : key.documentation.split("\n")) {

To fix this issue I suggest we change Field#description() to return a String like "No documentation available" or an empty String when description is not set/null.

I also suggest to open a bug ticket and fix it upstream in Kafka.