Handle null value of multi-value-properties

Hi Florian,

A first small contribution from the OpenDataSpace team (greetings from Tom Uhl)

This pull request contains 2 changes:

1. Fix handling of null values for Multi-Value-Properties (return an empty array - if the result is null)
2. Workaround for preventing a somewhat strange server-side Exception "Nothing wrong! Either this is a bug or threading..." which ocassionally gets thrown with latest apache-chemistry.

Cheers
 -Fritz