ProductPart SKU field editor vanishes after failed new content creation (OCC-232)

[sarahelsaig]

Describe the bug

If you forget to fill out the SKU field, it correctly returns with an "SKU must not be empty." error message, but then the input field vanishes so it's impossible to fix it and you have to recreate the content item from scratch.

To Reproduce

Steps to reproduce the behavior:

1. Go to Admin > Content > Content Items.
2. Click New > Product
3. Fill in any field except SKU.
4. Click the Publish button.
5. Observe the failure with the "SKU must not be empty." error message.
6. Observe the lack of an "SKU" field (screenshot below).

Expected behavior

1. There should be client side validation against empty SKU field.
2. If the server side validation fails, the SKU editor should still be displayed.

Screenshots

Jira issue