Fix EZP-23299: ezpm: adding a subtree with nodeId misses children

Link: https://jira.ez.no/browse/EZP-23299

## Description
The behavior was not the same when exporting content objects. When passing its path has parameter, it would get the whole subtree, and when passing its nodeId it would only get the children.

This patch makes both type of parameter retrieve the same content, meaning the whole subtree.

## Tests
Manual tests