Send an empty CSV file for invalid user segments

We were getting an exception in this case, which was OK I guess since
this shouldn't happen if the application is used in a normal way, but we
can simplify the code a little bit if we make the `recipients` code
return an empty list of users.

Note that the behavior of the `AdminNotification#list_of_recipients` and
`Newsletter#list_of_recipient_emails` methods is now slightly different;
previously they returned `nil` when given an invalid segment recipient,
while now they return an empty array. I haven't found a place where this
change is relevant. For example, in both of these models, the `deliver`
method used to raise an exception when given an invalid segment while
now it doesn't, but we always check the user segment is valid before
calling the `deliver` method anyway, so it doesn't really affect the
application.

app/models/admin_notification.rb

@@ -25,7 +25,7 @@ class AdminNotification < ApplicationRecord
   end
 
   def list_of_recipients_count
-    list_of_recipients&.count || 0
+    list_of_recipients.count
   end
 
   def deliver

app/models/newsletter.rb

@@ -11,7 +11,7 @@ class Newsletter < ApplicationRecord
   include ActsAsParanoidAliases
 
   def list_of_recipient_emails
-    UserSegments.user_segment_emails(segment_recipient) if valid_segment_recipient?
+    UserSegments.user_segment_emails(segment_recipient)
   end
 
   def valid_segment_recipient?