Remove empty emails from users segments recipients

Why:

User with an empty email value (nil) should not appear in the recipient
list for a given UserSegment at Newsletters or Email Downloads.

How:

Using Enumerable#compact and Enumerable#select to filter out empty emails

Increasing Email Download feature spec and Newsletter model spec to cover
all possible scenarios including the nil email one.

app/controllers/admin/emails_download_controller.rb

@@ -13,6 +13,6 @@ class Admin::EmailsDownloadController < Admin::BaseController
   private
 
   def users_segment_emails_csv(users_segment)
-    UserSegments.send(users_segment).newsletter.pluck(:email).to_csv
+    UserSegments.send(users_segment).newsletter.pluck(:email).compact.join(',')
   end
 end