config-schema incorrectly fails validation on optional props on objects

[gmmorris]

Kibana version:
7.7

Describe the bug:
I’ve encountered an issue in config-schema.
We have a key that’s optional on an object and for some reason, it doesn't pass validation.

Steps to reproduce:

Just put this in your test suite.

test('should allow optional nested objects', () => {
  const type = schema.object({
    name: schema.maybe(
      schema.object({
        type: schema.string(),
        id: schema.string(),
      })
    ),
  });

  expect(type.validate({})).toEqual({});
});

This test, weirdly, doesn’t pass.

It throws this:

[name.type]: expected value of type [string] but got [undefined]

But this one does:

test('should allow optional nested objects', () => {
  const type = schema.object({
    name: schema.maybe(schema.string()),
  });

  expect(type.validate({})).toEqual({});
});

Expected behavior:
Should pass validation.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)