Set parent button does not allow to remove a parent (make a page top-level)

The functionality added by NPMIG-9 allows to manually change the parent of a page in the screen displayed with the computed plan.

However, this functionality does not allow to completely remove the parent of a page, to render it top-level (allow it to be migrated to the root of the wiki). If the input for the parent is completely emptied and OK is clicked, nothing happens: it should save this value and make the page orphan,