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/sources/strategies/twitter.rb

@@ -41,15 +41,11 @@ module Sources::Strategies
 
     def image_urls
       if url =~ /(#{ASSET}[^:]+)/
-        return [$1 + ":orig" ]
-      elsif api_response.blank?
-        return [url]
-      end
-
-      [url, referer_url].each do |x|
-        if x =~ PAGE
-          return service.image_urls(api_response)
-        end
+        [$1 + ":orig" ]
+      elsif api_response.present?
+        service.image_urls(api_response)
+      else
+        [url]
       end
     end
     memoize :image_urls

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

app/views/uploads/_image.html.erb

@@ -1,7 +1,7 @@
 <% if params[:url] %>
   <% if ImageProxy.needs_proxy?(@source.image_url) %>
     <%= image_tag(image_proxy_uploads_path(:url => @source.image_url), :title => "Preview", :id => "image") %>
-  <% else %>
+  <% elsif @source.image_url.present? %>
     <%= image_tag(@source.image_url, :title => "Preview", :id => "image") %>
   <% end %>