Add null option to search filters

It can be difficult to find out where missing values occur or what those documents look like, since they cannot be filtered on directly.

Proposal: add a "select missing/empty values" option to each filter. Something like this:


It's pretty straightforward to extend the elasticsearch query for any filter, though doing so requires some changes to the backend which makes assumptions about how filters are formulated.

This would actually replace #1173 since you could filter on null values and then make a chart.