safekeeper: short-circuit interpreted wal sender (#10202)

## 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`.

safekeeper/src/send_interpreted_wal.rs

@@ -94,9 +94,14 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> InterpretedWalSender<'_, IO> {
                         }
                     }
 
+                    let max_next_record_lsn = match max_next_record_lsn {
+                        Some(lsn) => lsn,
+                        None => { continue; }
+                    };
+
                     let batch = InterpretedWalRecords {
                         records,
-                        next_record_lsn: max_next_record_lsn
+                        next_record_lsn: Some(max_next_record_lsn),
                     };
 
                     tx.send(Batch {wal_end_lsn, available_wal_end_lsn, records: batch}).await.unwrap();