Currently container_replace removes the container from the scratchpad
and re-adds it afterwards. For the split commands this results in the
container being send to the scratchpad, which results in a NULL segfault
if the same container should be shown.
Pass an optional workspace to root_scratchpad_add_container, if the
workspace is passed the window will continue to show on the workspace.
If NULL is passed it is sent to the scratchpad.
This was an issue if no other window except the scratchpad container was
on the workspace.
Fixes#4240
ddad41f423