Steps to reproduce:

1. Create MBean with CompositeData attribute (for example, fields "a", "b" and "c" are present in CompositeData, all of those are of String type).
2. Set value of field b to null.
3. Create JMX discovery.
4. Observe that JMX discovery will never get pass that attribute with the following error present in log: "failed: null".

Expected:
It is expected that null is treated as a valid value for a field.