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)
```

Start at lib/tod/time_of_day.rb around line 213 and inspect how the Rails multiparameter values are passed into Tod::TimeOfDay.new. Reproduce a case where one or more individual values are nil, then verify that initialization handles those values as proposed and no longer breaks.

Written by the indexing model from the issue text.