I18n messages backend does not support message metadata

[robhanlon22]

To Reproduce

# example.rb

require "i18n"
require "dry-schema"

I18n.backend.store_translations(
  :en,
  dry_schema: {
    errors: {
      key?: {
        text: "is missing",
        code: "123"
      }
    }
  }
)

schema = Dry::Schema.Params do
  config.messages.backend = :i18n
  config.messages.load_paths = []

  required(:id)
end

schema.call({}).errors[:key?]

$ ruby example.rb
Traceback (most recent call last):
	8: from example.rb:23:in `<main>'
	7: from /Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message_set.rb:79:in `[]'
	6: from /Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message_set.rb:67:in `to_h'
	5: from /Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message_set.rb:117:in `messages_map'
	4: from /Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message_set.rb:117:in `reduce'
	3: from /Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message_set.rb:117:in `each'
	2: from /Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message_set.rb:124:in `block in messages_map'
	1: from /Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message_set.rb:124:in `map!'
/Users/self/.rvm/gems/ruby-2.6.3/gems/dry-schema-1.4.1/lib/dry/schema/message.rb:55:in `dump': undefined method `empty?' for nil:NilClass (NoMethodError)

Expected behavior

Should not raise an error. Message should include the translated text as well as the metadata.

Your environment

• Affects my production application: NO
• Ruby version: 2.6.3
• OS: macOS 10.14.6