Site Editor: Return empty fragment from mobile areas while theme data loads

When areas.mobile is falsy, the layout briefly renders the desktop
sidebar instead of the mobile layout. Return an empty fragment rather
than null so the mobile layout container stays mounted during loading.

Co-authored-by: Claude <noreply@anthropic.com>

Author: dsas

packages/edit-site/src/components/site-editor-routes/home.js

@@ -30,7 +30,7 @@ export const homeRoute = {
 		},
 		mobile( { siteData } ) {
 			if ( ! isThemeDataLoaded( siteData ) ) {
-				return null;
+				return <></>;
 			}
 			return siteData.currentTheme.is_block_theme ||
 				isClassicThemeWithStyleBookSupport( siteData ) ? (

packages/edit-site/src/components/site-editor-routes/navigation-item.js

@@ -51,7 +51,7 @@ export const navigationItemRoute = {
 		},
 		mobile( { siteData } ) {
 			if ( ! isThemeDataLoaded( siteData ) ) {
-				return null;
+				return <></>;
 			}
 			return siteData.currentTheme.is_block_theme ? (
 				<MobileNavigationItemView />

packages/edit-site/src/components/site-editor-routes/navigation.js

@@ -45,7 +45,7 @@ export const navigationRoute = {
 		},
 		mobile( { siteData } ) {
 			if ( ! isThemeDataLoaded( siteData ) ) {
-				return null;
+				return <></>;
 			}
 			return siteData.currentTheme.is_block_theme ? (
 				<MobileNavigationView />

packages/edit-site/src/components/site-editor-routes/page-item.js

@@ -32,7 +32,7 @@ export const pageItemRoute = {
 		},
 		mobile( { siteData } ) {
 			if ( ! isThemeDataLoaded( siteData ) ) {
-				return null;
+				return <></>;
 			}
 			return siteData.currentTheme.is_block_theme ? (
 				<Editor />

packages/edit-site/src/components/site-editor-routes/pages.js

@@ -79,7 +79,7 @@ export const pagesRoute = {
 		},
 		mobile( { siteData } ) {
 			if ( ! isThemeDataLoaded( siteData ) ) {
-				return null;
+				return <></>;
 			}
 			return siteData.currentTheme.is_block_theme ? (
 				<MobilePagesView />

packages/edit-site/src/components/site-editor-routes/template-item.js

@@ -19,7 +19,7 @@ const areas = {
 	},
 	mobile( { siteData } ) {
 		if ( ! isThemeDataLoaded( siteData ) ) {
-			return null;
+			return <></>;
 		}
 		return siteData.currentTheme.is_block_theme ? (
 			<Editor />

packages/edit-site/src/components/site-editor-routes/templates.js

@@ -72,7 +72,7 @@ export const templatesRoute = {
 		},
 		mobile( { siteData } ) {
 			if ( ! isThemeDataLoaded( siteData ) ) {
-				return null;
+				return <></>;
 			}
 			if ( ! siteData.currentTheme.is_block_theme ) {
 				return <SidebarNavigationScreenUnsupported />;