Fix df plugin if all of Device, MountPoint, and FSType aren't set

The df plugin can work with just one of Device, MountPoint, and FSType set.
It will not however function with "None" in any of those fields, so just fix
it to output an empty string if there is no pillar value set.
Merge pull request #8 from iggy/for-upstream-fix-df-plugin

Fix df plugin if all of Device, MountPoint, and FSType aren't set