Allow filtering issues by any assignee (#33343)

This is the opposite of the "No assignee" filter, it will match all
issues that have at least one assignee.

Before
![Before
change](https://github.com/user-attachments/assets/4aea194b-9add-4a84-8d6b-61bfd8d9e58e)

After
![After change with any
filter](https://github.com/user-attachments/assets/99f1205d-ba9f-4a0a-a60b-cc1a0c0823fe)

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/org/projects.go

@@ -347,11 +347,11 @@ func ViewProject(ctx *context.Context) {
 	if ctx.Written() {
 		return
 	}
-	assigneeID := ctx.FormInt64("assignee") // TODO: use "optional" but not 0 in the future
+	assigneeID := ctx.FormString("assignee")
 
 	opts := issues_model.IssuesOptions{
 		LabelIDs:   labelIDs,
-		AssigneeID: optional.Some(assigneeID),
+		AssigneeID: assigneeID,
 		Owner:      project.Owner,
 		Doer:       ctx.Doer,
 	}