Restore empty filename check

Otherwise we could end up matching with a weird ".../.desktop" entry.

BUG: 427979


(cherry picked from commit a92ce693ec)

abstract_client.cpp

@@ -2616,6 +2616,10 @@ void AbstractClient::setDesktopFileName(QByteArray name)
 
 QString AbstractClient::iconFromDesktopFile() const
 {
+    if (m_desktopFileName.isEmpty()) {
+        return {};
+    }
+
     const QString desktopFileName = QString::fromUtf8(m_desktopFileName);
     QString desktopFilePath;