[Schema Inaccuracy] Issue `body_html` maybe `null`

[yanyongyu]

Schema Inaccuracy

As the body field of the Issue object schema, the issue body may be empty (null):

body:
  description: Contents of the issue
  type:
  - string
  - 'null'
  examples:
  - It looks like the new widget form is broken on Safari. When I try and
    create the widget, Safari crashes. This is reproducible on 10.8, but not
    10.9. Maybe a browser bug?

but the body_html and body_text field cannot be null in the schema. I tried to get some issue without body. The body_html and body_text of the issue is also null.

edited

Found other schemas with body_html and body_text should allow null:

• Issue
• Timeline Reviewed Event
• Release

Expected

The body_html and body_text field in the Issue schema should be string or null.

Reproduction Steps

curl -H "Accept: application/vnd.github.full+json" https://api.github.com/repos/nonebot/nonebot2/issues/1305