Sensor.h: 2012-05-04 Steven Lambright - Re-enabled a safety check in the...

Sensor.h: 2012-05-04 Steven Lambright - Re-enabled a safety check in the DemRadius() method which was needed due to Projection not uniformly handling Null inputs. Fixes #807. 

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4336 41f8697f-d340-4b68-9986-7bafba869bb8