[txn-emitter] Fix underflow when first request fails (#4567)

This debugging log was added to show txns around the failure, but when it is the first request, there is no previous transaction.

crates/transaction-emitter-lib/src/emitter/submission_worker.rs

@@ -332,7 +332,7 @@ pub async fn submit_transactions(
                             client
                                 .get_account_transactions_bcs(
                                     sender,
-                                    Some(account.into_inner().sequence_number() - 1),
+                                    Some(account.into_inner().sequence_number().saturating_sub(1)),
                                     Some(5),
                                 )
                                 .await