(Follow-up
r91059) Make backlinks query do both null or "" to potentially account for old rows that havn't been updated
yet to new conventions.
Hopefully I got the syntax right for the OR thing using mw's db functions (I tested the query, its fine, I'm just not 100% sure thats the most correct way using mw's db funcs, I've never really had to build an OR query using the db funcs).