[mob][photos] Bug fixes (#7426)

## Description

- Fix issue where deleting everything from a discover category causes
white screen.

## Tests

Tested in debug mode on my pixel phone.

Author: laurenspriem

mobile/apps/photos/lib/ui/viewer/gallery/device_folder_page.dart

@@ -56,7 +56,9 @@ class DeviceFolderPage extends StatelessWidget {
       header: Configuration.instance.hasConfiguredAccount()
           ? BackupHeaderWidget(deviceCollection)
           : const SizedBox.shrink(),
-      initialFiles: [deviceCollection.thumbnail!],
+      initialFiles: deviceCollection.thumbnail != null
+          ? [deviceCollection.thumbnail!]
+          : const <EnteFile>[],
     );
     return GalleryFilesState(
       child: Scaffold(

mobile/apps/photos/lib/ui/viewer/search/result/magic_result_screen.dart

@@ -165,7 +165,7 @@ class _MagicResultScreenState extends State<MagicResultScreen> {
       tagPrefix: widget.heroTag,
       selectedFiles: _selectedFiles,
       enableFileGrouping: _enableGrouping,
-      initialFiles: [files.first],
+      initialFiles: files.isNotEmpty ? [files.first] : const [],
     );
     return GalleryFilesState(
       child: InheritedSearchFilterDataWrapper(