Single checkbox invalid input on required validation

[robhuijben]

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
• Add terms of agreements checkbox code

  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.