Use qEnvironmentVariableIsEmpty

Instead of qgetenv("QT_AUTO_SCREEN_SCALE_FACTOR").isEmpty().
It's faster (don't allocate memory) and don't throw exceptions.

src/app/main.cpp

@@ -338,7 +338,7 @@ void showSplashScreen()
 #if defined(Q_OS_UNIX)
 void setupDpi()
 {
-    if (qgetenv("QT_AUTO_SCREEN_SCALE_FACTOR").isEmpty())
+    if (qEnvironmentVariableIsEmpty("QT_AUTO_SCREEN_SCALE_FACTOR"))
         qputenv("QT_AUTO_SCREEN_SCALE_FACTOR", "1");
 }
 #endif  // Q_OS_UNIX