First some food for thought.
The key thing about the feature is that we are using a special value in the target of a link to indicate that the source of the link has no other links.
It is not important what this special value is exactly, but it seemed appropriate to use (0,''), because it's simple, it doesn't require a schema change, and it's an invalid title and so won't appear otherwise.
The problem with it is that, as Brion hints, there are actually quite a lot of instances of this special value already present in the database. Here are the wikis with 10 or more:
dewiki 13456 enwiki 191 enwikinews 53 enwikiversity 12 huwiki 43 incubatorwiki 10 itwiki 15 kawiki 10 kuwiki 39 slwiki 15 zhwiki 40 itwikiversity 11
Now it's all very well to say "fix whatever bug is causing this", but that might not be practical, and is in fact not necessary for the feature in question. We can just use some other special invalid title, one that is more specific to this feature. For instance, we could use (-3,'##FROMBLANK'). It has a negative namespace which could be reserved in Defines.php as say NS_LINKHACK. And it starts with two "#" characters, so it doesn't conflict with any potential links from parser functions or fragments.
-- Tim Starling