Do not send empty batches with IRAC

It is possible for a site to send an empty batch to the remote site. Although it is harmless, the remote site creates an IntSet with zero capacity which is invalid.

The empty batch request generates an invalid reply which is not handled and the IracManager is not able to proceed sending new updates until the request timeout.

The following improvements can be made

• Avoid sending empty batches (waste of resources)
• Change IntSet to throw an exception with a zero capacity