def nrfa_metadata():
    """
    Return metadata on the NRFA data.

    Returned metadata is a dict with the following elements:

    - `url`: string with NRFA data download URL
    - `version`: string with NRFA version number, e.g. '3.3.4'
    - `published_on`: datetime of data release/publication (only month and year are accurate, rest should be ignored)
    - `downloaded_on`: datetime of last download

    :return: metadata
    :rtype: dict
    """
    result = {
        'url': config.get('nrfa', 'url', fallback=None) or None,  # Empty strings '' become None
        'version': config.get('nrfa', 'version', fallback=None) or None,
        'published_on': config.get_datetime('nrfa', 'published_on', fallback=None) or None,
        'downloaded_on': config.get_datetime('nrfa', 'downloaded_on', fallback=None) or None
    }
    return result