Correct getNodeTree

[ftomassetti]

This PR has been verified through a test in LionWeb Kotlin (see https://github.com/LionWeb-io/lionweb-kotlin/pull/1)

Changes:

• Documentation of the method. The method supports receiving multiple ids, but the documentation was stating otherwise
• I was getting an error when not specifying the clientId, and I think this was the intended behavior. I changed it in a way that providing no clientId should work, but providing somehow an invalid string should fail
• The way in which we were providing the answer was incorrect

Note that this PR is based on top of #76

[ftomassetti]

Thanks! I will press the "Rebase and merge" button then