fix: transaction manager service unmined lookup (#7192)

Description
---
Fixes sql lookup

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Refactor**
- Simplified internal query logic by removing redundant filter
conditions. No changes to user-facing features or functionality.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: SWvheerden

base_layer/wallet/src/transaction_service/storage/sqlite_db.rs

@@ -1862,6 +1862,7 @@ impl CompletedTransactionSql {
                     .or(completed_transactions::status.eq(TransactionStatus::CoinbaseConfirmed as i32)),
             )
             .filter(completed_transactions::mined_height.is_null())
+            .filter(completed_transactions::mined_height.eq::<Option<i64>>(None))
             .load::<CompletedTransactionSql>(conn)?)
     }