This looks like a partial regression of SERVER-5777

My test:
Index: {a:1, date:-1}
2.2.x query:

 
find({a:{$in:[1,2]}}).limit(5).sort({date:-1})  

correctly limits documents (nscanned and nscannedObject to 10).

find({a:{$in:[null,2]}}).limit(5).sort({date:-1})

same thing.

2.4.x query:

find({a:{$in:[1,2]}}).limit(5).sort({date:-1})

correctly limits to 10 docs.

find({a:{$in:[null,2]}}).limit(5).sort({date:-1})

scans all matching a documents without applying limit.