Related: What happened to FavoriteCount? - my educated guess is that a
NULL value indicates a question has never been favorited or bookmarked, and 0 that it was. Since the values are so different from what the column name conveys, shouldn't it just be dropped? That would break some old queries of course, but with developer tools it's often better to throw an exception rather than produce deceptive results.
ALTER TABLE Posts DROP COLUMN FavoriteCount - but I was afraid that would encourage people to post all kinds of SQL questions here ...)