error heroku staging with blank start_time

[Redcozmo]

issue with archived_courses method in person.rb

• make a migration to add start_date and end_date when its nil
• modify person methods archived_courses and pending_courses
• add validates on start_time and end_time

logs heroku :

2019-06-04T17:05:12.575585+00:00 app[web.1]: I, [2019-06-04T17:05:12.575476 #4] INFO -- : [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa] Completed 500 Internal Server Error in 11ms (ActiveRecord: 3.0ms)

2019-06-04T17:05:12.576266+00:00 app[web.1]: F, [2019-06-04T17:05:12.576208 #4] FATAL -- : [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa]

2019-06-04T17:05:12.576356+00:00 app[web.1]: F, [2019-06-04T17:05:12.576308 #4] FATAL -- : [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa] NoMethodError (undefined method `<' for nil:NilClass):

2019-06-04T17:05:12.576427+00:00 app[web.1]: F, [2019-06-04T17:05:12.576379 #4] FATAL -- : [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa]

2019-06-04T17:05:12.576501+00:00 app[web.1]: F, [2019-06-04T17:05:12.576459 #4] FATAL -- : [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa] app/models/person.rb:76:in `block in archived_courses'

2019-06-04T17:05:12.576504+00:00 app[web.1]: [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa] app/models/person.rb:75:in `each'

2019-06-04T17:05:12.576505+00:00 app[web.1]: [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa] app/models/person.rb:75:in `archived_courses'

2019-06-04T17:05:12.576507+00:00 app[web.1]: [1e8d5d5d-200e-495d-8c72-24bc7a7a3efa] app/controllers/courses_controller.rb:53:in `index'