feat: Allow null type for node on fireEvent and its shortcuts

A common pattern in some testing code is:

```js
const btn = document.getElementById('my-button')
fireEvent.click(btn)
```

However, `document.getElementById()` may return null and then TypeScript
will report an error:

```
error TS2345: Argument of type 'HTMLElement | null' is not assignable to parameter of type 'Document | Node | Element | Window'.
```

To resolve the TypeScript error, user can assert or check the returned
value wit something like:

```js
const btn = document.getElementById('my-button')
if (!btn) {
   throw new Error("Unable to find 'my-button')
}
fireEvent.click(btn)
```

But dom-testing-library is already doing this check, so let fireEvent
accept null as a convenience to call sites.

The error already reported by dom-testing-library is:

```
Unable to fire a "click" event - please provide a DOM element.
```

Author: jdufresne

src/__tests__/events.js

@@ -420,6 +420,18 @@ test('throws a useful error message when firing events on non-existent nodes (sh
   )
 })
 
+test('throws a useful error message when firing events on null', () => {
+  expect(() => fireEvent(null, new MouseEvent('click'))).toThrow(
+    'Unable to fire a "click" event - please provide a DOM element.',
+  )
+})
+
+test('throws a useful error message when firing events on null (shortcut)', () => {
+  expect(() => fireEvent.click(null)).toThrow(
+    'Unable to fire a "click" event - please provide a DOM element.',
+  )
+})
+
 test('throws a useful error message when firing non-events', () => {
   expect(() => fireEvent(document.createElement('div'), undefined)).toThrow(
     'Unable to fire an event - please provide an event object.',

types/__tests__/type-tests.ts

@@ -213,6 +213,10 @@ export function eventTest() {
   // Custom event
   const customEvent = createEvent('customEvent', element)
   fireEvent(element, customEvent)
+
+  // null returned by Document.getElementById()
+  const button = document.getElementById('my-button')
+  fireEvent.click(button)
 }
 
 export async function testWaitFors() {

types/events.d.ts

@@ -90,24 +90,24 @@ export type EventType =
   | 'online'
 
 export type FireFunction = (
-  element: Document | Element | Window | Node,
+  element: Document | Element | Window | Node | null,
   event: Event,
 ) => boolean
 export type FireObject = {
   [K in EventType]: (
-    element: Document | Element | Window | Node,
+    element: Document | Element | Window | Node | null,
     options?: {},
   ) => boolean
 }
 export type CreateFunction = (
   eventName: string,
-  node: Document | Element | Window | Node,
+  node: Document | Element | Window | Node | null,
   init?: {},
   options?: {EventType?: string; defaultInit?: {}},
 ) => Event
 export type CreateObject = {
   [K in EventType]: (
-    element: Document | Element | Window | Node,
+    element: Document | Element | Window | Node | null,
     options?: {},
   ) => Event
 }