Empty PR can be created through API

[siccous]

Description

Using the repoCreatePullRequest API endpoint you can create a PR which does not have any commits inside.

GitHub API disallows this action:

curl -L \
  -X POST \
  -H "Accept: application/vnd.github+json" \
  -H "Authorization: Bearer xxx" \
  -H "X-GitHub-Api-Version: 2022-11-28" \
  https://api.github.com/repos/xxx/xxx/pulls \
  -d '{"title":"Amazing new feature","body":"Please pull these awesome changes in!","head":"yyy:main","base":"main"}'

{
  "message": "Validation Failed",
  "errors": [
    {
      "resource": "PullRequest",
      "code": "custom",
      "message": "No commits between xxx:main and yyy:main"
    }
  ],
  "documentation_url": "https://docs.github.com/rest/pulls/pulls#create-a-pull-request",
  "status": "422"
}

My attempt to fix this is here #32742

Gitea Version

1.22.4

Can you reproduce the bug on the Gitea demo site?

Yes

Log Gist

No response

Screenshots

No response

Git Version

No response

Operating System

No response

How are you running Gitea?

selfhosted docker image v1.22.4

Database

None