fix(demo): handle empty user content preferences fixtures data

- Add a check for empty userContentPreferencesFixturesData before cloning the first item
- Throw a StateError if the fixtures data is empty, preventing potential runtime errors

Author: fulleni

lib/app/services/demo_data_initializer_service.dart

@@ -120,8 +120,14 @@ class DemoDataInitializerService {
       );
       // Clone the first item from the fixture data, assigning the new user's ID.
       // This provides new demo users with pre-populated saved filters and other preferences.
+      if (userContentPreferencesFixturesData.isEmpty) {
+        throw StateError(
+          'Cannot create preferences from fixture: userContentPreferencesFixturesData is empty.',
+        );
+      }
       final fixturePreferences = userContentPreferencesFixturesData.first
           .copyWith(id: userId);
+
       await _userContentPreferencesRepository.create(
         item: fixturePreferences,
         userId: userId,