FR: Make the backup_file optional when getting the config

[joewesch]

Currently when running the default get_config we need to send in a file path to backup_file so it can save the config to a file:

nornir-nautobot/nornir_nautobot/plugins/tasks/dispatcher/default.py

Lines 79 to 82 in 5f1236e

	make_folder(os.path.dirname(backup_file))
	with open(backup_file, "w", encoding="utf8") as filehandler:
	filehandler.write(running_config)

If we want to get the configuration but not save it we should be able to send in an empty string to disable this functionality.