Indexing an array can change query results

Creating an index on an array can cause documents to be omitted from query results.

Reproduction steps:
> doc =

{ _id : 1, arr : [] }

;

{ "_id" : 1, "arr" : [ ] }

> db.test.insert(doc);
> db.test.findOne(doc);

{ "_id" : 1, "arr" : [ ] }

> db.test.ensureIndex(

{ arr : 1 }

);
> db.test.findOne(doc);
null