Changed GetPositionFromGuid() to return a nullabe Vector3d

Modified the two places that currently call GetPositionFromGuid() to handle the null value instead of handling an exception. This is more in line with RemoteTechologiiesGroup "codiing style" to not throw execptions where possible.
The original version of this would have also failed if a bad target was passed, it would have failed with a unhandled ArgumentException, this version fails with a InvalidOperationException when it tries to reference `center.Value`.

The Debug.Assert code does not get compiled in to release code as it has a `ConditionalAttribute("DEBUG")` declared on it, it is only there to get rid of compiler warnings and to aid in debugging.