Default relative data path should be empty

Before, if no path was provided and `filename` was provided as an absolute
path, this would introduce absolute paths into the TeX output.
I.e. a very bad idea. Now, we just use the path given, it's the user's
responsibility to pick the right data path.
Merge branch 'fixRelativePath'

See also the discussion on 10aa60b2a696105dafdca8805bab7ad968d8877e
for the reason.
Tested in R2014b