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 `[]`.
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