fix deleting document data when using atomically without any operations

see example from my code. Some times where is no one record.set and in these cases record's data is completely destroyed with "update( {} )"

```
atomically do |record|
  fields_to_update.each do |field, value|
    next unless value_should_be_changed?(value)
    record.set(field, value)
  end
end
```