fix a bug from #30490
`prs.GetIssueIDs()` will also be used in other places, e.g.
`InvalidateCodeComments`
so we should not add `if pr.Issue == nil` in it, or if `pr.Issue` is
already loaded, you will not get the issueID in the results list and
this is not an expected result.
So this will caused a bug:
before calling `InvalidateCodeComments`, all `pr.Issues` in `prs` are
loaded, so `issueIDs` in this function will always be `[]`.
![image](https://github.com/go-gitea/gitea/assets/18380374/ef94d9d2-0bf9-455a-abd6-4d5e6497db7c)