assert.commandWorked(coll.insert([
    {_id: 1, a: 1, b: 1},
]));
coll.createIndex({c: 1, _id: 1});
assert.eq(coll.find({c: null}, {c: 1}).toArray(), [{_id: 1}]);

This is specific to Bonsai and is not an issue with the classic optimizer. Since we cannot tell just from the index whether the field `c` is missing or null, this requires an extra fetch. However Bonsai appears to append c: null to the resulting document. The test that exercises this is here.