An empty string for /apps/crebs/directories/slideshows in GConf prevents creation

[Marc Stewart]

If the directory name is deleted, CreBS is unable to save the slideshow.

Traceback (most recent call last):
  File "/usr/bin/../share/crebs/lib/crebs/gui.py", line 1213, in try_to_save
    self.save_slideshow()
  File "/usr/bin/../share/crebs/lib/crebs/gui.py", line 1332, in save_slideshow
    make_needed_dirs(self.output)
  File "/usr/bin/../share/crebs/lib/crebs/filesystem.py", line 32, in make_needed_dirs
    os.makedirs(s_dir)
  File "/usr/lib/python2.6/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 2] No such file or directory: ''

config.py should check for the empty string and replace it with the built-in default.