return nil for date sort by default, an empty string was causing an i…

…ndexing error

app/models/concerns/mods_data.rb

@@ -129,7 +129,7 @@ def mods_to_solr
   # 7. Partial date                    ["June 1781"]                   (eg: zq824dz1346)
   # 8. Approximate start and end years ["s. XIII^^ex [ca. 1275-1300]"] (eg: rc145sy7436)
   def parse_dates(input_dates)
-    dates = { all_int: [], sort: '' }
+    dates = { all_int: [], sort: nil }
     return dates if input_dates.blank?
     input_dates.each do |dt|
       dates[:all_int] << dt.to_s.scan(/[0-9]{3,4}/)
@@ -141,6 +141,7 @@ def parse_dates(input_dates)
     end
     dates[:all_int].flatten!
     unless dates[:all_int].blank?
+      dates[:sort] = ''
       dates[:all_int] = dates[:all_int].map(&:to_i)
       dates[:sort] = (dates[:all_int].inject { |sum, x| sum + x }) / dates[:all_int].length
     end