Return empty list instead of None

[jxltom]

Closes pypa/pipenv#3247

It is because None can not be iterated in

pythonfinder/src/pythonfinder/models/python.py

Line 70 in dd75de8

version_order = [versions[v] for v in parse_pyenv_version_order() if v in versions]

when version order file does not exist

[techalchemy]

Should we just check if the file exists first?

[jxltom]

It is already checked by os.path.exists(version_order_file), but this parse_pyenv_version_order will return None if file does not exist.

Actually even the order file does not exist, these functions should return empty list instead of None, otherwise for v in parse_pyenv_version_order() will raise error.

[techalchemy]

ok sounds good to me