Current implementation of Mongoid::Relations::Embedded::Many#concat will always run #batch_insert, even if the supplied docs array is empty. This could have a small performance impact on code relying on #concat but can also lead to an issue in Batchable#path where #atomic_path will be sent to nil.

This also brings the approach of #concat closer to the implementation of #<< where an empty list of arguments will leave the target untouched.