[Page] Fix subtitle not displaying if it's the only prop set (#7577)

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### WHY are these changes introduced?

If a <Page> component only has subtitle has a property, the
`hasHeaderContent` check will return false and no header will be
rendered.

This is especially valuable for Shopify Apps, where we will use the page
component to wrap our page, but will not make use of the `title` props
of the page, and instead leverage app-bridge's
[`TitleBar`](https://shopify.dev/apps/tools/app-bridge/actions/titlebar)

### WHAT is this pull request doing?

- Add a check for the existence of `subtitle` for defining
`hasHeaderContent` in `Page.tsx`
- Amend the unit test to reflect that the subtitle should render, even
if it's the only prop

### 🎩 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)
- [x] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [x] Updated the component's `README.md` with documentation changes
- [x] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: RickyMarou

.changeset/curvy-cats-burn.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Page component: display subtitle even when it's the only header prop set

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

@@ -33,6 +33,7 @@ export function Page({
 
   const hasHeaderContent =
     (rest.title != null && rest.title !== '') ||
+    (rest.subtitle != null && rest.subtitle !== '') ||
     rest.primaryAction != null ||
     (rest.secondaryActions != null &&
       ((isInterface(rest.secondaryActions) &&

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

@@ -58,7 +58,7 @@ describe('<Page />', () => {
   describe('subtitle', () => {
     it('gets passed into the <Header />', () => {
       const subtitle = 'Subtitle';
-      const page = mountWithApp(<Page {...mockProps} subtitle={subtitle} />);
+      const page = mountWithApp(<Page subtitle={subtitle} />);
       expect(page).toContainReactComponent(Header, {
         subtitle,
       });