gui: Show an error message if the restored wallet name is empty

The Restore Wallet dialog rejects wallet names that are empty, but was
doing so silently. This is confusing, we should be presenting an error
message to the user.

Author: achow101

src/qt/bitcoingui.cpp

@@ -452,7 +452,11 @@ void BitcoinGUI::createActions()
             //: Label of the input field where the name of the wallet is entered.
             QString label = tr("Wallet Name");
             QString wallet_name = QInputDialog::getText(this, title, label, QLineEdit::Normal, "", &wallet_name_ok);
-            if (!wallet_name_ok || wallet_name.isEmpty()) return;
+            if (!wallet_name_ok) return;
+            if (wallet_name.isEmpty()) {
+                QMessageBox::critical(nullptr, tr("Invalid Wallet Name"), tr("Wallet name cannot be empty"));
+                return;
+            }
 
             auto activity = new RestoreWalletActivity(m_wallet_controller, this);
             connect(activity, &RestoreWalletActivity::restored, this, &BitcoinGUI::setCurrentWallet, Qt::QueuedConnection);