trie: prefer nil slices over zero-length slices (#19084)

trie/database.go

@@ -809,7 +809,7 @@ func (db *Database) verifyIntegrity() {
 		db.accumulate(child, reachable)
 	}
 	// Find any unreachable but cached nodes
-	unreachable := []string{}
+	var unreachable []string
 	for hash, node := range db.dirties {
 		if _, ok := reachable[hash]; !ok {
 			unreachable = append(unreachable, fmt.Sprintf("%x: {Node: %v, Parents: %d, Prev: %x, Next: %x}",