"xa-datasource-properties" "Instance" property validation does not allow empty value

we have this property parametrised,

<xa-datasource-property name="Instance">
${sql.instance}
</xa-datasource-property>

so when ${sql.instance} is set to empty/blank/"", the startup falls over with the following error

17:55:00,435 ERROR [org.jboss.as.controller.management-operation] (ServerService Thread Pool – 26) Operation ("add") failed - address: ([
("subsystem" => "datasources"),
("xa-data-source" => "ImageGenAppsDS"),
("xa-datasource-properties" => "Instance")
]) - failure description: "JBAS014704: '' is an invalid value for parameter value. Values must have a minimum length of 1 characters"

This is a change of behaviour from 7.0.2.Final, and seems incorrect according to msdn references.