[Schema Inaccuracy] `head.label` and `head.user` can be null in `pull-request` schema

[xmo-odoo]

Schema Inaccuracy

The current schema for #/components/schemas/pull-request defines head.label and head.user as required.

Expected

head.label and head.user should have nullable: true with the nullability conditions documented (from what I understand, if the user's account was deleted for some reason: all the cases I can see are for "ghost" users)

Reproduction Steps

❯ curl -s https://api.github.com/repos/odoo/odoo/pulls/51976 | jq .head
{
  "label": null,
  "ref": "12.0",
  "sha": "3cf7aba2e293417c6e8a6a4785624c8315ccbb7a",
  "user": null,
  "repo": null
}

Side-note

The same issue was fixed for head.repo as reported in #161 and #424, would be nice to also add a description to that property explaining the conditions under which it can be null.

[cartalaba]

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