Return nil instead of an empty string from normalized_date (#1513)

lib/arclight/normalized_date.rb

@@ -43,7 +43,7 @@ def normalize
       result << inclusive if inclusive.present?
       result << other if other.present?
       result << "bulk #{bulk}" if bulk.present?
-      result.compact.map(&:strip).join(', ')
+      result.compact.map(&:strip).join(', ').presence
     end
   end
 end

spec/lib/arclight/normalized_date_spec.rb

@@ -84,7 +84,7 @@
       let(:date_other) { nil }
 
       it 'does not know what to do' do
-        expect(normalized_date).to eq ''
+        expect(normalized_date).to be_nil
       end
     end
   end