moebooru: fix preview_urls to fallback to image_urls.

Fix the moebooru strategy to fallback to returning the image url if we
can't find the preview url. Fixes iqdb lookups failing in some cases
because the strategy didn't return a valid url for preview_url.

app/logical/sources/strategies/moebooru.rb

@@ -62,7 +62,7 @@ module Sources
       end
 
       def preview_urls
-        return [] unless post_md5.present?
+        return image_urls unless post_md5.present?
         ["https://#{file_host}/data/preview/#{post_md5[0..1]}/#{post_md5[2..3]}/#{post_md5}.jpg"]
       end