Currently, if 'All' option is selected (for enumerated entries, when the criterion is displayed with a combobox), then the returned objects are the ones matching at least one option in the combobox
i.e. if 'a value entered by the user' is selected, it is a Solr OR clause between all the values which is requested
i.e. if 'a value entered by the user among' is selected, it a Solr OR clause between the values selected in the configuration of the service which is requested

But sometimes, a Content Attribute is not mandatory and is not filled for some contents, and therefore the Content is not retrievable if the criterion for this Attribute is selected

So sometimes it should be possible to "allow" objects with an empty value when the 'All' option is selected, and it should be configurable