getPullRequestInfo and getIssuesInfo indexed into the last element of an
empty Assignees slice when Action was HookIssueAssigned, causing a panic
under a race where the assignee list is empty by the time the webhook
payload is built.
Fixes https://github.com/go-gitea/gitea/issues/39826
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>