🍒ScreenPool: relax assertion in handleScreenAdded

When a new monitor is connected, the existing monitor might emit geometryChanged before qGuiApp emits screenAdded, but the new screen is already in qApp->screens(), so m_sizeSortedScreens still doesn't have the new screen at that point, which will make handleScreenAdded called twice.

SENTRY: PLASMA-WORKSPACE-29H BUG: 483432 FIXED-IN: 6.0.4

(cherry picked from commit 0692d762)