Fix bind-server CLI argument without --local-folder

$ python -m nxdrive bind-server USER URL --password PASSWORD
Exception:
Traceback (most recent call last):
  File "/home/tiger-222/projects/nuxeo-drive/nxdrive/__main__.py", line 114, in main
    return CliHandler().handle(sys.argv)
  File "/home/tiger-222/projects/nuxeo-drive/nxdrive/commandline.py", line 492, in handle
    return handler(options)
  File "/home/tiger-222/projects/nuxeo-drive/nxdrive/commandline.py", line 586, in bind_server
    check_credentials=check_credentials,
  File "/home/tiger-222/projects/nuxeo-drive/nxdrive/manager.py", line 538, in bind_server
    starts=start_engine,
  File "/home/tiger-222/projects/nuxeo-drive/nxdrive/manager.py", line 591, in bind_engine
    if not self.check_local_folder_available(local_folder):
  File "/home/tiger-222/projects/nuxeo-drive/nxdrive/manager.py", line 548, in check_local_folder_available
    if not local_folder.endswith("/"):
AttributeError: 'NoneType' object has no attribute 'endswith'