Can't drag tasks to parent tasks without subtasks

[raimund-schluessler]

#1136 unfortunately broke dragging tasks to parent tasks without subtasks. The reason is this change: https://github.com/nextcloud/tasks/pull/1136/files#diff-0429b1c69de468e1cf7ed2632ab1e520R23.

The subtasks container needs to be visible for the drag and drop to work.