Fix to check if device_path set to empty string

- Trove config does not allow the config option 'device_path' to be set to None.
- It has a default String value and setting an empty value in trove.conf will set the config option to empty string.

- This causes trove resize_flavor ( trove/trove/instance/models.py ) to throw an exception.

- Fix made to get device path to check for empty string as well