[5.4] Add empty array default to create method on belongs to many relationship

[calebporzio]

Belongs to many relationships are the only place the ->create([]) method doesn't have a default of empty array.

[mpociot]

This silently breaks packages that extend the BelongsToMany relationship class in a minor release!

Like laravel-mongodb:

https://github.com/jenssegers/laravel-mongodb/blob/master/src/Jenssegers/Mongodb/Relations/BelongsToMany.php#L94

Or the couchbase driver:

https://github.com/ORT-Interactive-GmbH/laravel-couchbase/blob/42a2fd6fcaa53a614900945ecf7895e1ac67ee23/src/Mpociot/Couchbase/Relations/BelongsToMany.php#L117

[antonkomarev]

Do it in Laravel 5.5, please.