bookmark: ensure tags sequence is always created

Somehow we are getting EphyBookmarks objects deserialized without initializing the tags property. I'm not sure how this happens. It even happens for JSON corresponding to bookmarks that definitely have tags set. Anyway, ephy_bookmark_get_tags() is used as if the result is not nullable, so let's guarantee this and return an empty list instead in this case.

This is a speculative fix for #612 (closed). It should fix the reported crash, but it's possible it will only uncover a subsequent crash.