do not facet null-valued properties; closes #69

facets are not created for null-valued properties in the following
sections:

- citation
- biological_context
- metadata

this change has the most affect on metadata, because the evidence
schema does not put constaints on null values.

lib/evidence/facet_filter.rb

@@ -31,11 +31,15 @@ def map_biological_context_facets(biological_context)
             value = annotation[:value]
             if value.respond_to?(:each)
               value.map { |v|
-                self.make_filter(:biological_context, name, v)
+                [:biological_context, name, v]
               }
             else
-              self.make_filter(:biological_context, name, value)
+              [:biological_context, name, value]
             end
+          }.select { |category, name, value|
+            value != nil
+          }.map { |filter|
+            self.make_filter(*filter)
           }
         else
           EMPTY
@@ -45,7 +49,11 @@ def map_biological_context_facets(biological_context)
       def map_metadata_facets(metadata)
         if metadata
           metadata.map { |name, value|
-            self.make_filter(:metadata, name, value)
+            [:metadata, name, value]
+          }.select { |category, name, value|
+            value != nil
+          }.map { |filter|
+            self.make_filter(*filter)
           }
         else
           EMPTY