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. It should fix the reported crash,
but it's possible it will only uncover a subsequent crash.