Handle sparse array positions as undefined V2

[VanTanev]

Superseeds #949; Fixes #676

Yup validation no longer throws on sparse arrays:

let schema = y.array(number())
let arr = []
arr[1] = 1
arr
// => [empty, 1]

// fixes TypeError: Cannot read property 'fulfilled' of undefined
schema.validate(arr, {
  abortEarly: false,
  context: {},
})

---

Yup required validation now treats empty slots in sparse arrays as missing:

let schema = y.array(y.number().required())
let arr = []
arr[1] = 1
arr
// => [empty, 1]

await schema.isValid(arr) // false
await schema.compact().isValid(arr) // true

---

Yup preserves empty slots in sparse arrays when valid:

let schema = y.array(y.number())
let arr = []
arr[1] = 1
arr
// => [empty, 1]

let value = await schema.validate(arr)

0 in value 
// => false

[jquense]

#949 (comment)

[jquense]

thanks!