[5.5] Allow to create relations without attributes

[antonkomarev]

Copy of #19498
Pointed it to 5.5 to not change signature of the method in minor updates as Taylor said.

I found it useful in one of my packages where I need to create related record but all attributes are optional. Right now we need to bypass an empty array to the relation create method:

$user->bans()->create([]);

With this change we will be able to omit attributes variable:

$user->bans()->create();

By the way create method calls Illuminate\Database\Eloquent::newInstance which has default value for $attributes already:

public function newInstance($attributes = [], $exists = false)