[Backdrop] Call onClick regardless of setClosing prop presence (#8237)

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### WHY are these changes introduced?

I believe #8090 introduced a
regression where the `onClick` prop of the `Backdrop` component is not
called when the consumer of the component also doesn't provide
`setClosing` as a prop (released in Polaris `10.23.0`). There are
various occurrences of Web Admin using `Backdrop` without providing
`setClosing` as a prop, so some sheets are not properly closing when
clicking outside. For example in Customer Segmentation:

<img width="1428" alt="Screenshot 2023-02-01 at 1 40 45 PM"
src="https://user-images.githubusercontent.com/3925905/216133586-92b760bf-57c2-4bfd-9f46-3382d8f935c2.png">

You can reproduce the issue in this
[CodeSandbox](https://codesandbox.io/s/condescending-cannon-7om9z7?file=/App.js:115-155).
Switching to a version of Polaris < `10.23.0` fixes the issue.

### WHAT is this pull request doing?

This PR ensures `onClick` is always called, regardless of `setClosing`
being provided.

### How to 🎩

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#local-development)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

<!--
  Give as much information as needed to experiment with the component
  in the playground.
-->

See Playground code. After opening the backdrop, clicking on it again
should properly close it.

<details>
<summary>Copy-paste this code in
<code>playground/Playground.tsx</code>:</summary>

```jsx
import React, {useState} from 'react';

import {Backdrop, Button, Page} from '../src';

export function Playground() {
  const [open, setOpen] = useState(false);

  return (
    <Page title="Playground">
      <div style={{ position: "absolute", zIndex: 999999 }}>
        <Button onClick={() => setOpen(!open)}>Toggle backdrop</Button>
      </div>
      {open && (
        <Backdrop
          setClosing={() => {}}
          onClick={() => {
            console.log("onClick from Backdrop");
            setOpen(false);
          }}
        />
      )}
    </Page>
  );
}

```

</details>

You can also 🎩 directly in Admin using this [Spin
instance](https://admin.web.customer-data-platform-ujet.loic-delaubier.us.spin.dev/store/shop1/customers):

- Go to the Customers section
- Open Segmentation templates
<img width="1189" alt="Screenshot 2023-02-01 at 2 18 18 PM"
src="https://user-images.githubusercontent.com/3925905/216141402-c395b9df-8990-4315-84ca-c88607b759fd.png">

- Click outside of the sheet. The sheet should properly close.

### 🎩 checklist

- [x] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [x] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: loic-d

.changeset/unlucky-bags-fry.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Fixed Backdrop onClick callback when setClosing is missing

polaris-react/src/components/Backdrop/Backdrop.tsx

@@ -30,8 +30,11 @@ export function Backdrop(props: BackdropProps) {
   };
 
   const handleClick = () => {
-    if (setClosing && onClick) {
+    if (setClosing) {
       setClosing(false);
+    }
+
+    if (onClick) {
       onClick();
     }
   };

polaris-react/src/components/Backdrop/tests/Backdrop.test.tsx

@@ -7,9 +7,16 @@ describe('<Backdrop />', () => {
   describe('onClick()', () => {
     it('is called when the backdrop is clicked', () => {
       const spy = jest.fn();
-      const backdrop = mountWithApp(
-        <Backdrop onClick={spy} setClosing={() => {}} />,
-      );
+      const backdrop = mountWithApp(<Backdrop onClick={spy} />);
+      backdrop.find('div')!.trigger('onClick');
+      expect(spy).toHaveBeenCalled();
+    });
+  });
+
+  describe('setClosing()', () => {
+    it('is called when the backdrop is clicked', () => {
+      const spy = jest.fn();
+      const backdrop = mountWithApp(<Backdrop setClosing={spy} />);
       backdrop.find('div')!.trigger('onClick');
       expect(spy).toHaveBeenCalled();
     });