def build_full_file_url(file_field, build_full_url):
    """
    Build the full URL for a file field.

    Args:
        file_field:
            The file field to build the full URL for.
        build_full_url:
            The function used to build a full URL out of an absolute
            path.

    Returns:
        The full URL to the file contained in the given field if it
        exists. If the field is empty, ``None`` is returned instead.
    """
    return build_full_url(file_field.url) if file_field else None