Default example breaks with dry-schema

The documented way of handling default values breaks using a struct in an array in `dry-schema`. When I remove this logic from my struct base class, there are no errors.

This is the example from the docs:

```ruby
class User < Dry::Struct
transform_types do |type|
if type.default?
type.constructor do |value|
value.nil? ? Dry::Types::Undefined : value
end
else
type
end
end

attribute :name, Types::String
attribute :age, Types::Integer.default(18)
end
```

This is what happens when you use the example above in a schema:

```ruby
Class.new(Dry::Validation::Contract) { params { required(:list).array(User) } }.new.call(list: [])
NoMethodError: undefined method `visit_key' for #
Did you mean? visit_any
visit_and
visit_sum
visit_lax
from /Users/joel/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/dry-types-1.4.0/lib/dry/types/predicate_inferrer.rb:55:in `public_send'
```

Removing either the `default` call or the `transform_types` prevents the error from happening.

I think I recall seeing an issue in the past where having the `constructor` call after the `default` call is an issue.

How would you recommend resolving this?

P.S. I'm using dry-struct 1.3.0

Start by reproducing the documented dry-struct example inside a dry-schema contract using required(:list).array(User), then inspect the failing dry-types-1.4.0/lib/dry/types/predicate_inferrer.rb:55 call to visit_key. Done means the schema accepts the array without raising NoMethodError while preserving default handling; the payload names no repository test file.

Written by the indexing model from the issue text.