Update PullRequest API assignees cannot be set to null

[viletyy]

Description

We can't use the api to set assignees to empty，but you can do so on our web page.

Screenshots

use API


use web

Gitea Version

v1.19.0-dev

Can you reproduce the bug on the Gitea demo site?

Yes

Operating System

No response

Browser Version

Google Chrome 版本 103.0.5060.53（正式版本） (arm64)