When creating an MBeanOperationInfo, the signature parameter, if null, is replace
by an impty MBeanParameterInfo[] array.

As a result, in our implementation, the final private signature field which is
serialized can *never* be null.

However, the fact that null is not a valid value is not explicitly documented in
the serial form.

An independent implementation could therefore choose to serialize a null value
instead of an empty array.
Our implementation should be made more robust and accept such cases.

With our current implementation, the JCK test javasoft.sqe.tests.api.javax.management.openmbean.OpenMBeanOperationInfoSupport.serial.InputTests serial2003 will fail if the deserialized signature field is null.