Composite primary keys and null values

[Tofandel]

Describe the bug

When inserting models with composite primary keys, if one of the keys is null, it's inserted with "$uid123" instead of the null value

Steps to reproduce the bug

1. Create a model with a composite primary key, one of which is nullable
2. Insert a model with a null primary key
3. Notice how it's been inserted at ["1","$uid1"]

Expected behaviour

The models to be inserted with the $id containing null values and not $uid1

Like this: ["1",null]

Versions

• Vuex ORM: 0.36.4
• Vue: 2.6.14

Link to minimal reproduction

https://codesandbox.io/s/vuex-orm-crud-sample-forked-qyc8u?file=/src/post.js

Additional context

I tried using a mutator setting null, while it fixes the retrieving, it doesn't solve the insertion
I also tried using user_id: this.uid(() => null), but it causes a different issue where the created $id is null (ignoring the fact that it's a composite key) if the value of that key is null