Fixes #1063. The MIN and MAX functions of the Calculator class were fixed so...

Fixes #1063. The MIN and MAX functions of the Calculator class were fixed so that they return a NULL value if either argument to them is a NULL value. This fixes an issue with the fx program.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4874 41f8697f-d340-4b68-9986-7bafba869bb8