Resolve "reminder_mailing show crashes if reminder mailing has no reminder_mailing_volunteer records"

Closes #438 (closed)

https://trello.com/c/HlvqxO8L/197-bug-remindermailing-show-crashes-if-reminder-mailing-has-no-remindermailingvolunteer-records

https://aoz-freiwillige.ch/reminder_mailings/8

View crashes because it tries to run method on nil here:

@reminder_mailing.reminder_mailing_volunteers.select(&:picked?).sample.process_template

show.html.slim Line 20

I, [2018-07-06T18:25:58.880003 #23560]  INFO -- : [28c90307-e98f-4843-9591-7dd26281ad9a]   Rendered reminder_mailings/show.html.slim within layouts/application (6.3ms)
I, [2018-07-06T18:25:58.880348 #23560]  INFO -- : [28c90307-e98f-4843-9591-7dd26281ad9a] Completed 500 Internal Server Error in 21ms (ActiveRecord: 2.7ms)
F, [2018-07-06T18:25:58.881603 #23560] FATAL -- : [28c90307-e98f-4843-9591-7dd26281ad9a]
F, [2018-07-06T18:25:58.881755 #23560] FATAL -- : [28c90307-e98f-4843-9591-7dd26281ad9a] ActionView::Template::Error (undefined method `process_template' for nil:NilClass):
F, [2018-07-06T18:25:58.882066 #23560] FATAL -- : [28c90307-e98f-4843-9591-7dd26281ad9a]     17: hr
[28c90307-e98f-4843-9591-7dd26281ad9a]     18:
[28c90307-e98f-4843-9591-7dd26281ad9a]     19: h3 Email Beispiel
[28c90307-e98f-4843-9591-7dd26281ad9a]     20: - example = @reminder_mailing.reminder_mailing_volunteers.select(&:picked?).sample.process_template
[28c90307-e98f-4843-9591-7dd26281ad9a]     21: h4 Betreff
[28c90307-e98f-4843-9591-7dd26281ad9a]     22: .mailing-body-preview= example[:subject]
[28c90307-e98f-4843-9591-7dd26281ad9a]     23: h4 Text
F, [2018-07-06T18:25:58.882265 #23560] FATAL -- : [28c90307-e98f-4843-9591-7dd26281ad9a]
F, [2018-07-06T18:25:58.882402 #23560] FATAL -- : [28c90307-e98f-4843-9591-7dd26281ad9a] app/views/reminder_mailings/show.html.slim:20:in `_app_views_reminder_mailings_show_html_slim___2746330942469394029_70322003135560'