Use default system time zone if it is not present in the file footer

That was existing behavior before ORC-341 was applied. Currently, if `useUTCTimestamp` is set in the reader, we consider the file was written in UTC if the timezone is not part of the file footer. To keep backwards compatibility for files that were written before HIVE-8746 was pushed, we should assume that the data was written in the system default time zone.