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/lib/user_segments.rb

@@ -66,6 +66,8 @@ class UserSegments
       all_users.where(geozone: geozones[segment.to_s])
     elsif valid_segment?(segment)
       send(segment)
+    else
+      User.none
     end
   end
 

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?

spec/controllers/admin/emails_download_controller_spec.rb

@@ -15,5 +15,12 @@ describe Admin::EmailsDownloadController do
       expect(response).to be_successful
       expect(response.body).to eq "admin@consul.dev,user@consul.dev"
     end
+
+    it "sends an empty file with an invalid users_segment" do
+      get :generate_csv, params: { users_segment: "invalid_segment" }
+
+      expect(response).to be_successful
+      expect(response.body).to be_empty
+    end
   end
 end

spec/lib/user_segments_spec.rb

@@ -307,7 +307,7 @@ describe UserSegments do
 
       recipients = UserSegments.recipients(:invalid_segment)
 
-      expect(recipients).to be nil
+      expect(recipients).to eq []
     end
 
     it "does not return any recipients when given a method name as a segment" do
@@ -315,7 +315,7 @@ describe UserSegments do
 
       recipients = UserSegments.recipients(:ancestors)
 
-      expect(recipients).to be nil
+      expect(recipients).to eq []
     end
   end
 
@@ -327,6 +327,14 @@ describe UserSegments do
       emails = UserSegments.user_segment_emails(:all_users)
       expect(emails).to eq ["first@email.com", "last@email.com"]
     end
+
+    it "returns an empty list when given an invalid segment" do
+      create(:user, email: "first@email.com")
+
+      emails = UserSegments.user_segment_emails(:invalid_segment)
+
+      expect(emails).to eq []
+    end
   end
 
   context "Geozones" do