eth/tracers: fix omitempty for memory and storage (#31289)

This fixes a regression in the opcode tracer API where we would log
empty memory and storage fields.

eth/tracers/logger/logger.go

@@ -177,14 +177,14 @@ func (s *StructLog) toLegacyJSON() json.RawMessage {
 	if len(s.ReturnData) > 0 {
 		msg.ReturnData = hexutil.Bytes(s.ReturnData).String()
 	}
-	if s.Memory != nil {
+	if len(s.Memory) > 0 {
 		memory := make([]string, 0, (len(s.Memory)+31)/32)
 		for i := 0; i+32 <= len(s.Memory); i += 32 {
 			memory = append(memory, fmt.Sprintf("%x", s.Memory[i:i+32]))
 		}
 		msg.Memory = &memory
 	}
-	if s.Storage != nil {
+	if len(s.Storage) > 0 {
 		storage := make(map[string]string)
 		for i, storageValue := range s.Storage {
 			storage[fmt.Sprintf("%x", i)] = fmt.Sprintf("%x", storageValue)