## Problem
Safekeeper may currently send a batch to the pageserver even if it
hasn't decoded a new record.
I think this is quite unlikely in the field, but worth adressing.
## Summary of changes
Don't send anything if we haven't decoded a full record. Once this
merges and releases, the `InterpretedWalRecords` struct can be updated
to remove the Option wrapper for `next_record_lsn`.