VRFS-1135 allow empty string to be passed in to start parameter

(due to jQuery not stripping null / empty params and refactor)

ruby/lib/jam_ruby/models/feed.rb

@@ -20,7 +20,7 @@ module JamRuby
       sort ||= 'date'
       raise "not valid sort #{sort}" unless SORT_TYPES.include?(sort)
 
-      start = params[:start]
+      start = params[:start].presence
       if sort == 'date'
         start ||= FIXNUM_MAX
       else