`DatasetVersion.release_date` can be empty and non-incremental

Just recording the observation

https://github.com/HumanBrainProject/fairgraph/issues/62

In addition, for https://kg.ebrains.eu/api/instances/a8932c7e-063c-4131-ab96-996d843998e9 the release date is non-incremental, ie. the date for v1.13 is after that for v1.18

This means that we cannot rely on this field to be accurate or even present unconditionally.