chore: don't panic on missing l2 batch number

src/processor/snapshot/exporter.rs

@@ -39,7 +39,7 @@ impl SnapshotExporter {
         let l2_batch_number = self
             .database
             .get_latest_l2_batch_number()?
-            .expect("db contains no l2 batch number");
+            .unwrap_or_default();
         let l2_block_number = self.database.get_latest_l2_block_number()?.unwrap_or({
             tracing::warn!("WARNING: the database contains no l2 block number entry and will not be compatible with the ZKSync External Node! To export a compatible snapshot, please let the prepare-snapshot command run until an l2 block number can be found.");
             U64::from(0)