[Page] Fix subtitle not displaying if it's the only prop set

.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,
       });