Custom sort for processes : server error when processes have empty / missing date

[moustachu]

Describe the bug
When the custom sort for processes is enabled (env var SORT_PROCESSES_BY_DATE=true), the front processes index return an HTTP error 500.
This need to be investigate but the probable cause is that existing processes have empty or missing start/end date.

To Reproduce
Steps to reproduce the behavior:

1. Enable Custom sort for processes with env var SORT_PROCESSES_BY_DATE=true
2. Check if there at least one process with an empty end date
3. Got to /processes
4. See error

Expected behavior
No error and the custom sort for processes enabled

Stacktrace
Application log

2024-10-30 14:46:42.480	F, [2024-10-30T13:46:42.480009 #1] FATAL -- : [fb777f2796a29e87a2c41a9e4d926f3f]   
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f] ActionView::Template::Error (comparison of Date with nil failed):
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]     25:     <%= render partial: "decidim/shared/participatory_space_filters/show" %>
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]     26:     <p class="loading"><%= t(".loading") %></p>
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]     27:     <div class="row small-up-1 medium-up-2 large-up-3 card-grid">
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]     28:       <%= render(collection) %>
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]     29:     </div>
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]     30:   </section>
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]     31: <% end %>
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f]   
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f] app/controllers/decidim/participatory_processes/participatory_processes_controller.rb:139:in `sort_by'
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f] app/controllers/decidim/participatory_processes/participatory_processes_controller.rb:139:in `custom_sort'
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f] app/controllers/decidim/participatory_processes/participatory_processes_controller.rb:96:in `participatory_processes'
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f] app/controllers/decidim/participatory_processes/participatory_processes_controller.rb:85:in `collection'
2024-10-30 14:46:42.480	[fb777f2796a29e87a2c41a9e4d926f3f] app/controllers/decidim/participatory_processes/participatory_processes_controller.rb:13:in `collection'

Additional context
see Pull request #596