Cannot create products with nullable customFields in admin UI

[alexisvigoureux]

Describe the bug

We can not create products with nullable customFields. Some of our customFields are optional in version 1.9 but do not work on 2.x.

To Reproduce
Steps to reproduce the behavior:

1. Add customFields

export const ProductCustomFields: CustomFieldConfig[] = [
{
    name: 'start_date',
    label: [
      {
        languageCode: LanguageCode.en,
        value: 'Start date',
      },
    ],
    type: 'datetime',
  },
{
    name: 'useType',
    type: 'string',
    nullable: true,
    label: [
      {
        languageCode: LanguageCode.fr,
        value: "Type d'utilisation du produit",
      },
      {
        languageCode: LanguageCode.en,
        value: 'Use type',
      },
    ],
    options: [
      {
        value: ProductUseType.EVERYWHERE,
        label: [
          {
            languageCode: LanguageCode.en,
            value: 'Usable everywhere',
          },
        ],
      },
      {
        value: ProductUseType.STORE,
        label: [
          {
            languageCode: LanguageCode.en,
            value: 'Usable in store',
          },
        ],
      },
      {
        value: ProductUseType.ONLINE,
        label: [
          {
            languageCode: LanguageCode.en,
            value: 'Usable online',
          },
        ],
      },
    ],
  },
]

3. Launch the server and go to "Inventory" in admin UI, click on "New product"
4. Fills in the product name and variants but leaves customFields empty
5. Click on the "Create" button
6. See error

• For start_date

Variable "$input" got invalid value {} at "input.customFields.start_date"; DateTime cannot represent non string or Date type {}

Even if we set nullable: true in the customFields, there's always an error because the graphql query sends "start_date": {}

• For useType

The custom field "useType" value [""] is invalid. Valid options are ['everywhere', 'offline', 'online']

The graphql query sends "useType": "" but the select is good

Expected behavior

We would like to have optional customFields with empty value, so we can create products without filling in the fields

Environment (please complete the following information):

• @vendure/core version: 2.0.6
• Nodejs version: 18.15
• Database (mysql/postgres etc): postgres

Additional context

[alexisvigoureux]

I tested with the 2.0.7 version. It fixed the datetime problem (start_date example) but not the nullable problem (useType example)