vmwaretools_version.rb - check for Nil result before running sub

I was getting this error on some hosts related to the vmwaretools_version fact:

> Puppet (err): Could not run Puppet configuration client: Could not retrieve local facts: private method `sub' called for nil:NilClass

Updated the fact to check for a value before running .sub.  Otherwise, set to "not installed".

FYI @craigwatson:  this is unrelated to the issue I had posted last week re: virtual vs. physical.