RF+BF: '' from `get_nibabel_data()` when missing

Return empty string from ``get_nibabel_data()`` if directory does not exist,
instead of None.

Doing this means we can use os.path.join on the output even if it the 
directory doesn't exist. The resulting filenames don't make sense, but the
decorator checks for this in any case, so the filenames don't get used.