Make empty dimensions_values_labels values invalid

For now, a dataset.json containing dimensions_values_labels key like:

  "dimensions_values_labels":{
    "Contry":{}
  },

is valid.

After this patch, this is not possible.

Details

To make empty elements invalid for this key, I added a new nonEmptyObjectOfLabels in definitions.json and used it in dataset.json.

Note

An empty dimensions_values_labels is still valid