Fix for null arrival and departure times in stats.xls

app/controllers/conferences_controller.rb

@@ -471,11 +471,11 @@ class ConferencesController < ApplicationController
 					@excel_data[:data] << {
 						:name => user.firstname,
 						:email => user.email,
-						:date => r.created_at.strftime("%F %T"),
+						:date => r.created_at ? r.created_at.strftime("%F %T") : nil,
 						:city => r.city,
 						:languages => ((JSON.parse(r.languages || '[]').map { |x| I18n.t"languages.#{x}" }).join(', ').to_s),
-						:arrival => r.arrival.strftime("%F %T"),
+						:arrival => (r.arrival || @this_conference.start_date).strftime("%F %T"),
-						:departure => r.departure.strftime("%F %T"),
+						:departure => (r.departure || @this_conference.end_date).strftime("%F %T"),
 						:housing => (I18n.t"articles.conference_registration.questions.housing.#{r.housing || 'none'}"),
 						:bike => (I18n.t"articles.conference_registration.questions.bike.#{r.bike || 'none'}"),
 						:food => (I18n.t"articles.conference_registration.questions.food.#{r.food || 'meat'}"),