Fix PullRequestList.GetIssueIDs's logic (#31352)

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)

Author: yp05327

models/issues/pull_list.go

@@ -192,8 +192,10 @@ func (prs PullRequestList) LoadIssues(ctx context.Context) (IssueList, error) {
 		return nil, nil
 	}
 
-	// Load issues.
-	issueIDs := prs.GetIssueIDs()
+	// Load issues which are not loaded
+	issueIDs := container.FilterSlice(prs, func(pr *PullRequest) (int64, bool) {
+		return pr.IssueID, pr.Issue == nil && pr.IssueID > 0
+	})
 	issues := make(map[int64]*Issue, len(issueIDs))
 	if err := db.GetEngine(ctx).
 		In("id", issueIDs).
@@ -229,10 +231,7 @@ func (prs PullRequestList) LoadIssues(ctx context.Context) (IssueList, error) {
 // GetIssueIDs returns all issue ids
 func (prs PullRequestList) GetIssueIDs() []int64 {
 	return container.FilterSlice(prs, func(pr *PullRequest) (int64, bool) {
-		if pr.Issue == nil {
-			return pr.IssueID, pr.IssueID > 0
-		}
-		return 0, false
+		return pr.IssueID, pr.IssueID > 0
 	})
 }