ProductVariant updates not persisted when custom field relation defined

[michaelbromley]

Describe the bug
When a "relation" custom field is defined on ProductVariant, updating with the updateProductVariants mutation will return the old values of direct properties (e.g. stockOnHand, sku). Fields which rely on a related entity (name, price) are correctly updated.

To Reproduce
Steps to reproduce the behavior:

1. Define a relation custom field on ProductVariant
2. In the AdminUI, edit a variant, e.g. increase the stock.
3. Save, and then refresh the page
4. See that the stock has reverted to the original value.

Expected behavior
Updates should work

Environment (please complete the following information):

• @vendure/core version: v1.1.1+
• Nodejs version: any
• Database (mysql/postgres etc): any

Additional context
I think it is related to this commit from v1.1.1: 510025a