Always display subtitle in Page

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

With this commit we add the possiblility to render a header inside a Page
if it only has `subtitle` set as a prop.
We also amend the test to reflect that.

Author: RickyMarou

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,
       });