Bug fix: isDrive "" == False

isDrive is only called from `dropTrailingPathSeparator` and `combineAlways`.
Both times occur after a check if the argument is not empty (i.e. null for
combineAlways, and hasTrailingPathSeparator for dropTrailingPathSeparator). So
this change is safe.
Merge pull request #17 from thomie/fix-isDrive-empty

Bug fix: isDrive "" == False