bug(Navisworks): Collections with no elements *successfully* converting to null

[jsdbroughton]

Collections generated from Navis node conversion convert successfully, but where they have no elements existing behaviour omits them from returning the converted object, and instead, the Converted Objects list carries it as an Element with a null Speckle representation.

This, in turn, causes the recombination of the parent-child hierarchy builder post-conversion/pre-commit to fail.