root_for_each_container and root_find_container were using incorrect
logic to determine if a container was hidden in the scratchpad.
Containers will have a NULL parent if they are a direct child of a
workspace. Containers will have a NULL workspace if they are hidden in
the scratchpad.
The incorrect check meant that root_for_each_container would run the
callback on scratchpad containers twice. This meant that executing a
command such as `[class="$something"] scratchpad show` would cause the
command to run twice, resulting in the container being shown and hidden
again which is effectively a no op.
Fixes#2655.
c6ff1f67f1