minor (related to STANBOL-878): ValueTypeSerializerRegistry now checks if the Service aquired for the ServiceReference parsed to addService is NULL to avoid possible NPE. While this avoid NPE in the logs it not a functional change as OSGI correctly recovers from such NPE - hence no need for an own issue

git-svn-id: https://svn.apache.org/repos/asf/stanbol/trunk@1551157 13f79535-47bb-0310-9956-ffa450edef68