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>

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)
 		}