Don't wrap arrays twice: `children` are already wrapped in an array

We do the wrapping in an array in represent_hierarchy for children.

app/serializers/group_child_serializer.rb

@@ -27,8 +27,8 @@ class GroupChildSerializer < BaseSerializer
     if children.is_a?(GroupDescendant)
       represent_hierarchy(children.hierarchy(hierarchy_root), opts).first
     else
-      hierarchies = Array.wrap(GroupDescendant.merge_hierarchies(children, hierarchy_root))
-      hierarchies.map { |hierarchy| represent_hierarchy(hierarchy, opts) }.flatten
+      hierarchies = GroupDescendant.merge_hierarchies(children, hierarchy_root)
+      represent_hierarchy(hierarchies, opts)
     end
   end