Fix #3984: Twitter: undefined method `first' for nil:NilClass.

Fix Sources::Strategies::Twitter#image_urls to return an empty array
instead of nil when the tweet doesn't contain any images.

app/logical/twitter_service.rb

@@ -67,6 +67,8 @@ class TwitterService
       extract_urls_for_status(tweet)
     elsif tweet.urls.any?
       extract_urls_for_card(tweet)
+    else
+      []
     end
   end
 end