def is_version_dev_valid(dotted_ver_dev):
    """
    Checks if the development marker is valid.

    Args:
      dotted_ver_dev (str): The dev marker portion of the version string to
        check.  This should have already stripped the plus (`+`) sign that needs
        to immediately precede it.

    Returns:
      (bool or None): True if the dev marker is present and is the correct
        format; False if there is text but it is the wrong format; None if the
        string is empty.
    """
    if not dotted_ver_dev:
        return None

    if dotted_ver_dev.lower() == 'dev':
        return True

    return False