fix: oldest unpicked batch (#692)

## What ❔

Set sealed batch number if no unpicked job were found

## Why ❔

Sometimes there is no such jobs in db, so it remains the same.

## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
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- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
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core/lib/zksync_core/src/house_keeper/fri_prover_queue_monitor.rs

@@ -91,13 +91,24 @@ impl PeriodicJob for FriProverStatsReporter {
 
         let mut db_conn = self.db_connection_pool.access_storage().await.unwrap();
 
-        if let Some(l1_batch_number) = db_conn
+        let oldest_unpicked_batch = match db_conn
             .proof_generation_dal()
             .get_oldest_unpicked_batch()
             .await
         {
-            metrics::gauge!("fri_prover.oldest_unpicked_batch", l1_batch_number.0 as f64)
-        }
+            Some(l1_batch_number) => l1_batch_number.0 as f64,
+            // if there is no unpicked batch in database, we use sealed batch number as a result
+            None => {
+                db_conn
+                    .blocks_dal()
+                    .get_sealed_l1_batch_number()
+                    .await
+                    .unwrap()
+                    .unwrap()
+                    .0 as f64
+            }
+        };
+        metrics::gauge!("fri_prover.oldest_unpicked_batch", oldest_unpicked_batch);
 
         if let Some(l1_batch_number) = db_conn
             .proof_generation_dal()