fix(tasks): include tasks with deleted parents in subtask-expanded queries

When expand[]=subtasks is used, the LEFT JOIN on task_relations filters
out same-project subtasks. If a parent task was deleted, the JOIN
produces NULL for parent_tasks.id/project_id, and since NULL != value
evaluates to NULL (not TRUE) in SQL, these tasks were incorrectly
excluded.

Add builder.IsNull{"parent_tasks.id"} to the OR condition so tasks
whose parent was deleted are still included in results.

pkg/models/task_search.go

@@ -369,6 +369,7 @@ func (d *dbTaskSearcher) Search(opts *taskSearchOptions) (tasks []*Task, totalCo
 	if expandSubtasks {
 		cond = builder.And(cond, builder.Or(
 			builder.IsNull{"task_relations.id"},
+			builder.IsNull{"parent_tasks.id"},
 			builder.Expr("parent_tasks.project_id != tasks.project_id"),
 		))
 	}