Router should accept arrays or primitives a body payloads

[pgayvallet]

The validation changes in #51919 seems to have broken the actual data pre-validation, and wrongly coerce anything to objects, causing any primitive or array send as post payload to convert to an empty object :

Sample test that fails:

  it('accept to receive an array payload', async () => {
    const { server: innerServer, createRouter } = await server.setup(setupDeps);
    const router = createRouter('/');

    let body: any = null;
    router.post(
      {
        path: '/',
        validate: {
          body: schema.arrayOf(schema.object({ foo: schema.string() })),
        },
      },
      (context, req, res) => {
        body = req.body;
        return res.ok({ body: 'ok' });
      }
    );
    await server.start();

    await supertest(innerServer.listener)
      .post('/')
      .send([{ foo: 'bar' }, { foo: 'dolly' }])
      .expect(200);

    expect(body).toEqual([{ foo: 'bar' }, { foo: 'dolly' }]);
  });

expected 200 "OK", got 400 "Bad Request"

with the actual error message being

[request body.0]: expected a plain object value, but found [Array] instead

Culprit seems to be:

kibana/src/core/server/http/router/validator/validator.ts

Lines 269 to 279 in 3bdbcd0

	private preValidateSchema(data: any) {
	if (Buffer.isBuffer(data)) {
	// if options.body.parse !== true
	return schema.buffer();
	} else if (data instanceof Stream) {
	// if options.body.output === 'stream'
	return schema.stream();
	} else {
	return schema.maybe(schema.nullable(schema.object({}, { allowUnknowns: true })));
	}
	}

[elasticmachine]

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