Fix case where email method short-circuits and returns nil

Changes.md

@@ -1,6 +1,7 @@
 HEAD
 -----------
 
+- ActionMailer.delay.method now only tries to deliver if method returns a valid message.
 - Logging now uses "MSG-#{Job ID}", not a random msg ID
 - Allow generic Redis provider as environment variable. [#443]
 - Add ability to customize sidekiq\_options with delay calls [#450]

lib/sidekiq/extensions/action_mailer.rb

@@ -16,7 +16,10 @@ module Sidekiq
 
       def perform(yml)
         (target, method_name, args) = YAML.load(yml)
-        target.send(method_name, *args).deliver
+        msg = target.send(method_name, *args)
+        # The email method can return nil, which causes ActionMailer to return
+        # an undeliverable empty message.
+        msg.deliver if msg && msg.to && msg.from
       end
     end