If number of items in a Solr facet is zero, do not display the facet in the UI

[tchaddad]

This is especially relevant when we filter results by source, then by category. For example:

• if we select data from NOAA Economics (http://wcga-vm01.sdsc.edu/discover/#?s=NOAA%20Economics:%20National%20Ocean%20Watch), we expect the results to be mostly in the Human categories, and yet, when we look at the Categories being displayed for further filtering, we see all possible categories, even those with zero contents.

This is not ideal. Only facets with contents that are relevant (non-zero) should be visible for filtering.

[tchaddad]

I should add that hiding of "zero" facets appears to be working as expected if the user starts filtering by Issue or Category. However the behavior is different if the user begins to filter by first choosing a Source. If the user starts with Source, the zero facet items are not removed from the UI.

This elevates this issue to bug status.