QIcon::setIsMask: delete extra judgment

QIcon::detach can judgment if the d pointer is nullptr

Pick-to: 6.1
Change-Id: I90fd5f50ed2565a5654b978c4603635e62677953
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/image/qicon.cpp

@@ -1377,8 +1377,7 @@ bool QIcon::hasThemeIcon(const QString &name)
 */
 void QIcon::setIsMask(bool isMask)
 {
-    if (d)
-        detach();
+    detach();
     if (!d)
         d = new QIconPrivate(new QPixmapIconEngine);
     d->is_mask = isMask;