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

[fishytodd]

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.

WC-registry Ecotrust/wc-data-registry#124

[tchaddad]

As mentioned in the original issue, the 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.

[tchaddad]

To duplicate the problem behavior follow these steps:

• start a new Discover session : http://portal.westcoastoceans.org/discover/
• begin to filter by first choosing a SOURCE, e.g.: USGS
• once a source is selected (any source), open CATEGORIES and view list

You will see that the full list of CATEGORIES is presented, with most items listing zero. The optimal behavior would be to only show CATEGORIES that have non-zero values.