We plan to make changes to the validation of optional parameters.
As it stands, if a centre submits an optional parameter with a blank value/empty string etc, a warning is given but the value is accepted.
Going forward, if the tag exists for an optional parameter, it will be validated as any other parameter would be, so blank/empty values will be failed.
If you do not intend to submit an optional parameter, the tag must be omitted from the XML.
A new version of the Validation API has already been released containing this new rule so that you can start preparing for the change.