message: refactor: Use bilingual_str::empty()
diff: @@ -262,7 +262,7 @@ void CreateWalletActivity::finish()
  {
  destroyProgressDialog();
  - if (!m_error_message.original.empty()) {
  + if (!m_error_message.empty()) {
  QMessageBox::critical(m_parent_widget, tr("Create wallet failed"), QString::fromStdString(m_error_message.translated));
  } else if (!m_warning_message.empty()) {
  QMessageBox::warning(m_parent_widget, tr("Create wallet warning"), QString::fromStdString(Join(m_warning_message, Untranslated("\n")).translated));
  @@ -303,7 +303,7 @@ void OpenWalletActivity::finish()
  {
  destroyProgressDialog();
  - if (!m_error_message.original.empty()) {
  + if (!m_error_message.empty()) {
  QMessageBox::critical(m_parent_widget, tr("Open wallet failed"), QString::fromStdString(m_error_message.translated));
  } else if (!m_warning_message.empty()) {
  QMessageBox::warning(m_parent_widget, tr("Open wallet warning"), QString::fromStdString(Join(m_warning_message, Untranslated("\n")).translated));
  