refacto: username is nullable

integration_test/duniter/docker-compose.yml

integration_test/scenarios/gecko_complete.dart

lib/providers/substrate_sdk.dart

lib/providers/wallet_options.dart

lib/providers/wallets_profiles.dart

lib/screens/search.dart

lib/screens/transaction_in_progress.dart

lib/screens/wallet_view.dart

lib/widgets/contacts_list.dart

lib/widgets/history_view.dart

lib/widgets/payment_popup.dart

lib/widgets/search_identity_query.dart

lib/widgets/search_result_list.dart

lib/widgets/transaction_tile.dart