Return an empty size if no suitable entry found to avoid mismatch with
the returned pixmap()'s size (the QIconEngine::actualSize() returns the
originally requested size).
Change-Id: Ia278719a54392b62c5f9fc0529476baba5cd7df0
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>