Querying all_committed shouldn't get timestamp zero

I saw the following error when running test format locally:

[1553832348:988869][20492:0x7f7e74fe1700], t, WT_CONNECTION.open_session: __wt_txn_parse_timestamp, 147: Failed to parse oldest timestamp '0': zero not permitted: Invalid argument

The oldest timestamp is queried from all_committed and not checked - we should update that code to not work if all_committed returns 0