[libssu] Dont use QString.arg() with an empty string. Fixes JB#51400

Previously we tried to replace the username placeholder in "home-url"
even when the homeUrl was empty this resulted in messages like this
when trying to register via `ssu r`:

Username: user
Password:
DBus unavailable, falling back to libssu
QString::arg: Argument missing: , user
Operation successful (direct)

Signed-off-by: Björn Bidar <bjorn.bidar@jolla.com>

libssu/ssu.cpp

@@ -490,11 +490,11 @@ void Ssu::sendRegistration(const QString &usernameDomain, const QString &passwor
     manager->post(request, form.query(QUrl::FullyEncoded).toStdString().c_str());
     // we could expose downloadProgress() from reply in case we want progress info
 
-    QString homeUrl = settings->value("home-url").toString().arg(username);
+    QString homeUrl = settings->value("home-url").toString();
     if (!homeUrl.isEmpty()) {
         // clear header, the other request bits are reusable
         request.setHeader(QNetworkRequest::ContentTypeHeader, 0);
-        request.setUrl(homeUrl + "/authorized_keys");
+        request.setUrl(homeUrl.arg(username) + "/authorized_keys");
         ssuLog->print(LOG_DEBUG, QString("Trying to get SSH keys from %1").arg(request.url().toString()));
         pendingRequests++;
         manager->get(request);