Don't warn "interleaved frame-sync and actual events" if DA is null

Amends 1285b67a11

It might turn out that this warning is just noise in other cases too;
not sure.  But it's quite normal that currentEventDeliveryAgent is null
when the once-per-frame fake hover event occurs.

Fixes: QTBUG-94576
Pick-to: 6.2
Change-Id: Ib4eed274801b08b62403caec70d0fc3c538e4383
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

src/quick/util/qquickdeliveryagent.cpp

@@ -1430,7 +1430,8 @@ void QQuickDeliveryAgentPrivate::flushFrameSynchronousEvents(QQuickWindow *win)
         qCDebug(lcHoverTrace) << q << "frame-sync hover delivery done";
     }
 #endif
-    if (Q_UNLIKELY(QQuickDeliveryAgentPrivate::currentEventDeliveryAgent != q))
+    if (Q_UNLIKELY(QQuickDeliveryAgentPrivate::currentEventDeliveryAgent &&
+                   QQuickDeliveryAgentPrivate::currentEventDeliveryAgent != q))
         qCWarning(lcPtr, "detected interleaved frame-sync and actual events");
     QQuickDeliveryAgentPrivate::currentEventDeliveryAgent = deliveringAgent;
 }