Refactor generate_different_date_within_collection_year to handle edge cases and return nil if no available dates

app/helpers/collection_time_helper.rb

@@ -57,6 +57,10 @@ module CollectionTimeHelper
   def generate_different_date_within_collection_year(date, start_date_override: nil, end_date_override: nil)
     start_date = [start_date_override, collection_start_date(date).to_date].compact.max.to_date
     end_date = [end_date_override, collection_end_date(date).to_date].compact.min.to_date
-    ((start_date..end_date).to_a - [date.to_date]).sample
+
+    return nil if start_date > end_date
+
+    available_dates = (start_date..end_date).to_a - [date.to_date]
+    available_dates.empty? ? nil : available_dates.sample
   end
 end