update nested attribute didn't check for nil

https://github.com/mongoid/mongoid/blob/master/lib/mongoid/relations/builders/nested_attributes/many.rb#L102

This line doc will be nil if the id passed in does not exist, should've have a nil check to catch appropriate errors and queue it in active record errors object.

Related to: https://github.com/mongoid/mongoid/issues/2484 because it is still ocurring