[Schema Inaccuracy] Pull Request Simple Head repo label field may be null

[bigfootjon]

Schema Inaccuracy
The Pull Request Simple object field head.repo may be null if user has deleted the repo, as the Pull Request object.

schemas:

incorrect

pull-request-simple:
type: object
title: Pull Request Simple
properties:
head:
type: object
properties:
repo:
"$ref": "#/components/schemas/repository"

correct

pull-request:
type: object
title: Pull Request
properties:
...
head:
type: object
properties:
repo:
type:
- object
- 'null'
Expected
change the repo field to oneof repository / type: null

Reproduction Steps
list pull request using curl -X GET https://api.github.com/repos/apache/pulsar/pulls?state=open

get pull request apache/pulsar#6036 which head repo was deleted

Related to yanyongyu/githubkit#6

[bigfootjon]

Previously filed as #1811

[bigfootjon]

The label field can also be null if the branch is deleted according to yanyongyu/githubkit#56