KWin 5 ScreenShotDBusInterface2: Fix valid capture area with 0 width or height
QRect::isEmpty is only true when size is negative, unlike QSize::isEmpty and QRectF::isEmpty. Use QRectF, QSizeF or QSize to check if a rectangle is empty, not QRect.
The KWin 5 counterpart to !4351 (closed) since it's no longer possible to cherry-pick KWin effect commits from master to Plasma/5.27.