Incorrect processing of nil value for array with nested structure

## Describe the bug

I get a `KeyError` when pass the `nil` value for option, but it's ok when the option is omitted

## To Reproduce

```ruby
class X
extend Dry::Initializer

option :fields, [], optional: true, default: proc { [] } do
option :name
end
end

X.new #=> #
X.new(fields: []) #=> #
X.new(fields: nil) #=> KeyError (X::Fields: option 'name' is required)
```

## Expected behavior

Getting and empty array

## My environment

- Ruby version: 2.7.5
- OS: Linux
- dry-initializer (3.1.1)