fix(content_management): handle null values in DraftSourcesState

- Update DraftSourcesState.copyWith constructor to handle null values
- Ensure lastPendingDeletionId and snackbarSourceTitle retain their current
  values if null is passed

Author: fulleni

lib/content_management/bloc/draft_sources/draft_sources_state.dart

@@ -82,8 +82,9 @@ class DraftSourcesState extends Equatable {
       // to ensure they are cleared after being handled.
       exception: exception,
       publishedSource: publishedSource,
-      lastPendingDeletionId: lastPendingDeletionId,
-      snackbarSourceTitle: snackbarSourceTitle,
+      lastPendingDeletionId:
+          lastPendingDeletionId ?? this.lastPendingDeletionId,
+      snackbarSourceTitle: snackbarSourceTitle ?? this.snackbarSourceTitle,
     );
   }