Do not wait for timer if there are no open windows

If there were no open windows KWin would wait until the timer ran
out and return false signifying that some windows didnt close.
BUG:477573
FIXED-IN:6.0

src/sm.cpp

@@ -446,6 +446,13 @@ bool SessionManager::closeWaylandWindows()
             toplevelWindow->closeWindow();
         }
     }
+
+    if (m_pendingWindows.empty()) {
+        m_closingWindowsGuard.reset();
+        QDBusConnection::sessionBus().send(dbusMessage.createReply(true));
+        return true;
+    }
+
     m_closeTimer.start(std::chrono::seconds(10));
     m_closeTimer.setSingleShot(true);
     connect(&m_closeTimer, &QTimer::timeout, m_closingWindowsGuard.get(), [this, dbusMessage] {