Nyx -d ignores supplied file name

Thanks for this nice tool!

Running nyx from git without arguments destroys the interface with stem debug messages. One needs to

$ ./run_nyx 2>nyx.log

The offered -d option seems to fail reliably:

$ ./run_nyx -d nyx.log
Traceback (most recent call last):
  File "./run_nyx", line 14, in <module>
    nyx.main()
  File "nyx/nyx/__init__.py", line 176, in main
    nyx.starter.main()
  File "/usr/lib/python2.7/dist-packages/stem/util/conf.py", line 289, in wrapped
    return func(*args, config = config, **kwargs)
  File "nyx/nyx/starter.py", line 67, in main
    _setup_debug_logging(args)
  File "nyx/nyx/starter.py", line 135, in _setup_debug_logging
    os.makedirs(debug_dir)
  File "/usr/lib/python2.7/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 2] No such file or directory: ''

The packaged version says

$ nyx -d nyx.log
Unable to write to our debug log file (nyx.log): No such file or directory

$ nyx --version
nyx version 2.0.4 (released November 5, 2017)