Bug: `create_core_instance` raises `TypeError` for `config_file=None`

[samhitech]

Config_file None raises TypeError when checking for relative paths

For config_file=None, create_core_instance raises TypeError when attempting to convert config_file from a relative to an absolute path.

if not os.path.isabs(config_file):
        config_file = os.path.join(mm_app_path, config_file)

Fixed the issue by checking if config_file is None before checking if it is a relative path in PR #22.