def _tags_from_list(tags_list):
    """Return the list of tags from a list of tag parameter values.

    Each parameter value in the list may be a list of comma separated tags, with empty strings
    ignored.
    """
    tags = []
    if tags_list is not None:
        for tag in tags_list:
            tags.extend([t for t in tag.split(",") if t != ""])
        return tags
    return None