Empty data are stored in the repository with a specific value (_infinity+1 for long values for exemple)

This way, we can make the differnce between empty or not present values. But the JCR requests, not using the data holder API but the Expression onesee these specific values as normal ones and a NotExpression will saay that a data is present even if it is empty. A MetadataExpression will return the specific value (when the data holder API returns null)

We decided to change the storage, rather than migrate the Expression API.

When a data is present but empty, set an ametys-internal metadata for this data to specify that it is present but empty. This solution is prefered as an only propoerty containing a list of empty data, because complex data (using nodes instead of properties) already use a system with a boolean metadata. The list then will contain some of the emtpy data but not the other ones