This attribute has been erroneously set to be required in the schema.
Setting this to 'optional' in the schema will prevent validation errors on data when it is not required. This is a bug, and this fix is backwards compatible. As it has been discovered as part of the 1.04 process we feel it is possible to include it in the upgrade - even though it was not flagged at the start of the process.
If there are objections to the inclusion of this fix in this upgrade, please let us know.
Technical details can be found at https://github.com/IATI/IATI-Schemas/pull/61