Allow to "change" structure type on node with no previous structure

Allow a structure type to be set on nodes with no previous structure.

Also removed the `NodeHelper` introduced by @chirimoya as we can do this better in the `SuluNodeHelper`.
I will make a PR on jackalope to introduce this optimization into the core, but I copied this logic into the relevant method in the `SuluNodeHelper` as this is the only place we need it right now.