Check Event constructor params

[justjavac]

fix 2 bugs:

1. Event requires at least 1 argument
2. Convert the parameter type to string

---

Chrome:

const event = new Event()
// TypeError: Failed to construct 'Event': 1 argument required, but only 0 present.

Firefox:

const event = new Event()
// TypeError: Event requires at least 1 argument, but only 0 were passed

Deno(Before fixed):

const event = new Event()
event.type === undefined

after fixed:

const event = new Event()
// TypeError: Event requires at least 1 argument, but only 0 present

---

Before fixed:

const event = new Event(null)
event.type === null  // true

After fixed:

const event = new Event(null)
event.type === null    // false
event.type === "null"  // true

[ry]

LGTM - thanks!