Fix insertion id handling in TreebankRetriever

The changes made in #451 also need action in the Retriever to work
properly!
More carefulness when persisting morph info

We don't want to add 'undefined' as a string - this can especially
happen with artificialTokens
Merge pull request #452 from latin-language-toolkit/at_fixes2

Temporarily fix for faulty artificialToken handling on a Perseids Backend II