Fix empty project displayed in issue sidebar (#25802)

Before:

![image](https://github.com/go-gitea/gitea/assets/18380374/1ab476dc-2f9b-4c85-9e87-105fc73af1ee)
After:

![image](https://github.com/go-gitea/gitea/assets/18380374/786f984d-5c27-4eff-b3d9-159f68034ce4)

This issue comes from the change in #25468.
`LoadProject` will always return at least one record, so we use
`ProjectID` to check whether an issue is linked to a project in the old
code.
As other `issue.LoadXXX` functions, we need to check the return value
from `xorm.Session.Get`.

In recent unit tests, we only test `issueList.LoadAttributes()` but
don't test `issue.LoadAttributes()`. So I added a new test for
`issue.LoadAttributes()` in this PR.

---------

Co-authored-by: Denys Konovalov <privat@denyskon.de>

3 files changed, 51 insertions, 7 deletions

diff --git a/models/issues/issue_list_test.go b/models/issues/issue_list_test.go
index 97ce9e43b3..696c3b765d 100644
--- a/models/issues/issue_list_test.go
+++ b/models/issues/issue_list_test.go
@@ -67,9 +67,7 @@ func TestIssueList_LoadAttributes(t *testing.T) {
 		if issue.ID == int64(1) {
 			assert.Equal(t, int64(400), issue.TotalTrackedTime)
 			assert.NotNil(t, issue.Project)
-		} else if issue.ID == int64(2) {
-			assert.Equal(t, int64(3682), issue.TotalTrackedTime)
-			assert.Nil(t, issue.Project)
+			assert.Equal(t, int64(1), issue.Project.ID)
 		} else {
 			assert.Nil(t, issue.Project)
 		}
diff --git a/models/issues/issue_project.go b/models/issues/issue_project.go
index b163c68357..782638d997 100644
--- a/models/issues/issue_project.go
+++ b/models/issues/issue_project.go
@@ -16,13 +16,14 @@ import (
 func (issue *Issue) LoadProject(ctx context.Context) (err error) {
 	if issue.Project == nil {
 		var p project_model.Project
-		if _, err = db.GetEngine(ctx).Table("project").
+		has, err := db.GetEngine(ctx).Table("project").
 			Join("INNER", "project_issue", "project.id=project_issue.project_id").
-			Where("project_issue.issue_id = ?", issue.ID).
-			Get(&p); err != nil {
+			Where("project_issue.issue_id = ?", issue.ID).Get(&p)
+		if err != nil {
 			return err
+		} else if has {
+			issue.Project = &p
 		}
-		issue.Project = &p
 	}
 	return err
 }
diff --git a/models/issues/issue_test.go b/models/issues/issue_test.go
index 80699a57b4..7f1eab1971 100644
--- a/models/issues/issue_test.go
+++ b/models/issues/issue_test.go
@@ -17,6 +17,7 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 
 	"github.com/stretchr/testify/assert"
@@ -539,3 +540,47 @@ func TestCountIssues(t *testing.T) {
 	assert.NoError(t, err)
 	assert.EqualValues(t, 18, count)
 }
+
+func TestIssueLoadAttributes(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	setting.Service.EnableTimetracking = true
+
+	issueList := issues_model.IssueList{
+		unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 1}),
+		unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 4}),
+	}
+
+	for _, issue := range issueList {
+		assert.NoError(t, issue.LoadAttributes(db.DefaultContext))
+		assert.EqualValues(t, issue.RepoID, issue.Repo.ID)
+		for _, label := range issue.Labels {
+			assert.EqualValues(t, issue.RepoID, label.RepoID)
+			unittest.AssertExistsAndLoadBean(t, &issues_model.IssueLabel{IssueID: issue.ID, LabelID: label.ID})
+		}
+		if issue.PosterID > 0 {
+			assert.EqualValues(t, issue.PosterID, issue.Poster.ID)
+		}
+		if issue.AssigneeID > 0 {
+			assert.EqualValues(t, issue.AssigneeID, issue.Assignee.ID)
+		}
+		if issue.MilestoneID > 0 {
+			assert.EqualValues(t, issue.MilestoneID, issue.Milestone.ID)
+		}
+		if issue.IsPull {
+			assert.EqualValues(t, issue.ID, issue.PullRequest.IssueID)
+		}
+		for _, attachment := range issue.Attachments {
+			assert.EqualValues(t, issue.ID, attachment.IssueID)
+		}
+		for _, comment := range issue.Comments {
+			assert.EqualValues(t, issue.ID, comment.IssueID)
+		}
+		if issue.ID == int64(1) {
+			assert.Equal(t, int64(400), issue.TotalTrackedTime)
+			assert.NotNil(t, issue.Project)
+			assert.Equal(t, int64(1), issue.Project.ID)
+		} else {
+			assert.Nil(t, issue.Project)
+		}
+	}
+}