Custom field with entity ProductVariant throws Cannot set properties of null (setting 'taxCategory') when value is empty

[andriinuts]

Describe the bug
When you are adding a custom field as ProductVariant entity it throws an error ``Cannot set properties of null (setting 'taxCategory')` when you are leaving this value empty (for example when creating a new order line).

config.customFields.OrderLine.push({ name: 'relatedVariant', type: 'relation', entity: ProductVariant, });

To Reproduce
Steps to reproduce the behavior:

1. Go to the new draft order and add a new item without selecting relatedVariant value
2. Click Add item to order
3. See error

Expected behavior
I would expect that we will not receive this error because the field it not required in the configuration and it should be possible to skip it.

Environment (please complete the following information):

• @vendure/core version: 2.1.8
• Nodejs version: v20.10
• Database (mysql/postgres etc): any