Use default on record filter when given an empty string

[solebared]

Thanks for fixing #412 in v4, this really does help with using AI for form objects.

It appears however that 5a5337c did not cover record filters? Perhaps this was intentional, but i think treating a blank/empty input string as nil could also apply here.

For example, say we have a form for creating an Issue,

class IssueForm < ActiveInteraction::Base
  record :assignee, class: Contributor, default: nil
  # ...
end

where assignee is selected with a dropdown on the frontend, but is optional.

When the user omits the assignee, params would still include assignee: ''.
Passing params directly into the interactor would then fail:

IssueForm.run!(assignee: '')  #=> ActiveInteraction::InvalidInteractionError (Assignee is not a valid record)

I think it would be intuitive for AI to assign the default value here, as is the case with other filters.

[solebared]

Attempted to also demonstrate this with a unit test in this commit, though i'm not confident this is the right place to specify the behavior i'm proposing.

[AaronLasseigne]

This will be in the 5.0.0 release.