Empty hashes not detected by validate_keys is true

[patodevilla]

Describe the bug

When setting validate_keys as true, empty hashes are being ignored by the validator

To Reproduce

schema = Dry::Schema.JSON do
  config.validate_keys = true
end
schema.({hello: {}})
 => #<Dry::Schema::Result{} errors={}> 
schema.({hello: 1})
 => #<Dry::Schema::Result{} errors={:hello=>["is not allowed"]}>

Expected behavior

Unexpected key hello should be in the errors hash regardless of the key's value

My environment

• Affects my production application: YES
• Ruby version: ruby-2.5.7