fix pushing to n-to-n in a embedded in doc will raise nil error when not assign parent

situation has described in #3019

we could return a empty hash when not assign a parent, but there is a problem is ```atomic_paths``` and ```atomic_selector``` are cached and will not change whatever association changing.
e.g
```ruby
2.0.0p353 :012 >   a1 = A.create
 => #<A _id: 52a6450a4a6173a58b000000, >
2.0.0p353 :013 > a2 = A.create
 => #<A _id: 52a6450a4a6173a58b010000, >
2.0.0p353 :014 >
2.0.0p353 :015 >   b = B.new
 => #<B _id: 52a6450b4a6173a58b020000, c_ids: nil>
2.0.0p353 :016 > b.a = a1
 => #<A _id: 52a6450a4a6173a58b000000, >
2.0.0p353 :017 > b.atomic_path
 => "b"
2.0.0p353 :018 > b.atomic_selector
 => {"_id"=>BSON::ObjectId('52a6450a4a6173a58b000000'), "b._id"=>BSON::ObjectId('52a6450b4a6173a58b020000')}
2.0.0p353 :019 > a1
 => #<A _id: 52a6450a4a6173a58b000000, >
2.0.0p353 :020 > b.a = a2
 => #<A _id: 52a6450a4a6173a58b010000, >
2.0.0p353 :021 > b.atomic_path
 => "b"
2.0.0p353 :022 > b.atomic_selector
 => {"_id"=>BSON::ObjectId('52a6450a4a6173a58b000000'), "b._id"=>BSON::ObjectId('52a6450b4a6173a58b020000')}
2.0.0p353 :023 > a2
 => #<A _id: 52a6450a4a6173a58b010000, >
```
so I think we need a smarty way to caching them