CLI import failure if no username is passed.

Summary

CLI username default is set to None if --username flag is not passed. As a result, the import_name import directory creation fails.

Reproduction

$ memoryfm import spotify ~/Downloads/Streaming_History_Audio_2025_2.json
....
....
/home/siddhant/Documents/test_venv/.venv/lib/python3.13/site-packages/memoryfm/cli/utils/_import_utils.py:20 in create_import_name_dir

> 20     import_name_dir = imports_dir / name
TypeError: unsupported operand type(s) for /: 'PosixPath' and 'NoneType'