UX is inconsistent when filter value is left blank. The intuitive experience is that a blank value would leave the filter out of the current search. With some query types, this already works. But with several others, a blank value leads to zero search results. This is actually a back end issue: infrastructure for complex query building should be checking for the blank value on all query types. At the front end, this should be accompanied by a "Clear" link consistent with the UX we already have in Facet Widgets.