JSON schema contract failing to recognise valid input

## Describe the bug

A json schema that is not picking up on valid input.

## To Reproduce

```
class TestContract < Dry::Validation::Contract
json do
required(:mykey?).value(:bool)
end
end

input = JSON.parse('{"mykey?": true }')

result = TestContract.new.call(input)
# #["is missing"]}>

result.to_h
# {}

result.errors
# #] options={}>
```

## Expected behavior

It should have picked up the input since it was conforming to the schema.

## My environment

- Affects my production application: **NO**
- Ruby version: 2.6.10
- OS: macOS 12.6