[SERVER-6226] findAndModify return value inconsistent between 2.0.x and 2.1.2 when using upsert - MongoDB

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Found this while testing PyMongo against 2.1.2. In 2.0.x mongod returns {'ok': 1.0, 'value': {}}, in 2.1.2 it returns

{'ok': 1.0, 'value': null}

MongoDB shell version: 2.0.6

connecting to: test

> db.test.findAndModify({query: {'_id': 1}, update: {'$inc': {'i': 1}}, upsert: true})

{ }

MongoDB shell version: 2.1.2

connecting to: test

> db.test.findAndModify({query: {'_id': 1}, update: {'$inc': {'i': 1}}, upsert: true})

null

This will cause problems for code that expects a document as return value.