Single checkbox invalid input on required validation

### Problem
When a form contains a single checkbox with required validation, the server side validation returns an 'invalid input' message and the form cannot be submitted.

### Steps to reproduce
* Clean install Grav CMS 1.7.27.1
* Create form with [the example contact form code](https://learn.getgrav.org/16/forms/forms/example-form)
* Add terms of agreements checkbox code

```agree_to_terms:
type: checkbox
label: "Agree to the terms and conditions"
validate:
required: true
```

* Fill in the form in the frontend, check the agreements checkbox
* Validation never passes and message 'Invalid input in Agree to the terms and conditions' is displayed

### Expected result
Validation should pass when checkbox is checked and form should be submitted.

Start by reproducing the issue in Grav CMS 1.7.27.1 with the example contact form and the shown required checkbox configuration, then trace the server-side validation handling for a single checked checkbox. Done means the checked checkbox passes required validation and the form submits without the “Invalid input” message.

Written by the indexing model from the issue text.