Liferay's OSGi mechanisms will typically only work within an OSGi container. Most of the static util classes use an OSGi ServiceTracker to track when the implementation is created and registered.
You're getting an NPE because there was no registration, no instance available for the service tracker to wire in and make available, so when the util class goes to use the instance that it expects to be there, it isn't and you get an NPE.
Instead of using the util class, you should probably instantiate an implementation directly. Note that it will also be your responsibility to satisfy any dependencies it has (and all dependencies the dependencies might have).
In my opinion, if you're outside of a Liferay context, you'll probably be better off leveraging other frameworks outside of Liferay's. GSON or the standard JSON implementations are both good libs that provide similar functionality.
