Fix empty project displayed in issue sidebar (#25802) (#25854)

Backport #25802 by @yp05327

You can confirm this issue in
https://try.gitea.io/yp05327/testrepo/issues/2

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: yp05327 <576951401@qq.com>
Co-authored-by: Denys Konovalov <privat@denyskon.de>

routers/web/org/projects.go

@@ -437,8 +437,7 @@ func UpdateIssueProject(ctx *context.Context) {
 	projectID := ctx.FormInt64("id")
 	for _, issue := range issues {
 		if issue.Project != nil {
-			oldProjectID := issue.Project.ID
-			if oldProjectID == projectID {
+			if issue.Project.ID == projectID {
 				continue
 			}
 		}

routers/web/repo/projects.go

@@ -386,8 +386,7 @@ func UpdateIssueProject(ctx *context.Context) {
 	projectID := ctx.FormInt64("id")
 	for _, issue := range issues {
 		if issue.Project != nil {
-			oldProjectID := issue.Project.ID
-			if oldProjectID == projectID {
+			if issue.Project.ID == projectID {
 				continue
 			}
 		}