def iso_datetime_optional(timestamp):
if timestamp:<EOL><INDENT>return iso_datetime(timestamp)<EOL><DEDENT>return "<STR_LIT>"<EOL>
Convert UNIX timestamp to ISO datetime string, or "never".

        @param timestamp: UNIX epoch value.
        @return: Timestamp formatted as "YYYY-mm-dd HH:MM:SS", or "never" for false values.