Exception handling for Rails multiparam attribute

Looks like this change introduced a bug

https://github.com/jackc/tod/blob/master/lib/tod/time_of_day.rb#L213

In some cases, values for individual values can be `nil` which breaks the initialization.

I propose adding default values if any of the results are undefined

```ruby
::Tod::TimeOfDay.new(time_of_day[4] || 0, time_of_day[5] || 0, time_of_day[6] || 0)
```