QGenericUnixServices: avoid dbus calls on non xdg envs

Fixes: QTBUG-130884
Pick-to: 6.8 6.5
Change-Id: I2525293f776676cda63e61c3c5093045dbafcf38
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 1969b1a90a79dad049694bd1540f8627d5f26875)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/platform/unix/qgenericunixservices.cpp

@@ -373,6 +373,9 @@ private:
 
 QGenericUnixServices::QGenericUnixServices()
 {
+    if (desktopEnvironment() == QByteArrayLiteral("UNKNOWN"))
+        return;
+
 #if QT_CONFIG(dbus)
     if (qEnvironmentVariableIntValue("QT_NO_XDG_DESKTOP_PORTAL") > 0) {
         return;