Setting a relationship to `null` does not allow it to be set to any other value afterwards

## Version

v3.15.0

## Expected Behavior

When setting a relationship to `null`, you should then be able to set it back to another value.

## Actual Behavior

When setting a relationship to `null`, you are not able to set its value to anything else and it remains set to `null`.

## Steps to reproduce

```JavaScript
this.changeset.relationship = this.store.peekRecord('relationship', id); // value is set to the found record.
this.changeset.relationship = null; // value is set to null.
this.changeset.relationship = this.store.peekRecord('relationship', id); // value continues to be set to null

```