[FIX] Twitter image wasn't displayed properly, the existence of the topic...

[FIX] Twitter image wasn't displayed properly, the existence of the topic image wasn't tested properly

Twitter image was not seen (https://cards-dev.twitter.com/validator) so I changed the metatag syntax accordingly. Featured image metatag was added to a article_image (if found) or topic_image (even if empty), now only featured image is used if it exist.(priority) Even if they were no image topic the metatag was added for an article. Now we test there is an image (filename is not empty). Added comments for clarity.

Tested on many cases as possible;