add(exporter): sanity check for el payload exporter

backend/pkg/exporter/modules/execution_payloads_exporter.go

@@ -1,6 +1,7 @@
 package modules
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
 	"fmt"
@@ -154,9 +155,26 @@ func (d *executionPayloadsExporter) maintainTable() (err error) {
 	if err != nil {
 		return fmt.Errorf("error processing blocks: %w", err)
 	}
+	// sanity checks: check if any block hashes are 0x0000000000000000000000000000000000000000000000000000000000000000 or duplicate, check if count matches expected
+	if uint64(len(resData)) != maxBlock-minBlock+1 {
+		return fmt.Errorf("error processing blocks: expected %v blocks, got %v", maxBlock-minBlock+1, len(resData))
+	}
+	seen := make(map[string]bool)
+	emptyBlockHash := bytes.Repeat([]byte{0}, 32)
+	for _, r := range resData {
+		if len(r.BlockHash) == 0 {
+			return fmt.Errorf("error processing blocks: block hash is empty")
+		}
+		if bytes.Equal(r.BlockHash, emptyBlockHash) {
+			return fmt.Errorf("error processing blocks: block hash is all zeros")
+		}
+		if _, ok := seen[string(r.BlockHash)]; ok {
+			return fmt.Errorf("error processing blocks: duplicate block hash")
+		}
+		seen[string(r.BlockHash)] = true
+	}
 
 	// update the execution_payloads table
-
 	log.Infof("preparing copy update to temp table")
 
 	// load data into temp table