Simplify Option[Seq[_]] -> Seq[_] once validation for arrays is done for govuk-frontend

[dabd]

Once this issue is resolved alphagov/govuk-frontend#1618 (there is branch with a resolution here https://github.com/dorightdigital/govuk-frontend/commits/identify-empty-arrays), we can simplify components that take lists of things Option[Seq[_]] into Seq[_]. This type was introduced to map as a Scala type Javascript's undefined and empty arrays which are respectively falsy and truthy values.
Ex:

The input to govukDateInput becomes:

items: Seq[InputItem]      = Nil,

instead of

items: Option[Seq[InputItem]]      = None,

And the template:

@dateInputItems = @{
  items.getOrElse(Seq(
    InputItem(name = "day", classes = "govuk-input--width-2"),
    InputItem(name = "month", classes = "govuk-input--width-2"),
    InputItem(name = "year", classes = "govuk-input--width-4")
  ))
}

can become

@dateInputItems = @{
  if (items.nonEmpty) items
  else
    (Seq(
      InputItem(name = "day", classes = "govuk-input--width-2"),
      InputItem(name = "month", classes = "govuk-input--width-2"),
      InputItem(name = "year", classes = "govuk-input--width-4")
    ))
}

The same changes should be applied to all the affected components and this will be a breaking change.