Check for 'None' string in accredited value when creating tooltip text

'None' is actually a string in the data, not the python value of
None so this if statement was always returning true. Now
we explicitly check for the 'None' string.

I don't really like the idea of having a value that represents 'None'
that isn't null or empty so we may want to rethink how data is being
loaded and checks being performed, but there really isnt a point in
doing that until we have the actual data in case it changes.
Merge pull request #314 from notthatbreezy/topic/icon-tooltip-text-fix

Check for 'None' string in accredited value when creating tooltip text