Added additional safeguards on accessors for Line, Sample, and Pixel Shift...

Added additional safeguards on accessors for Line, Sample, and Pixel Shift returning Null if any value in a requisite computation would be Null

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2805 41f8697f-d340-4b68-9986-7bafba869bb8