Fix setPlaceholderText in QLineEdit

Since we don't use focus to show/hide place holder text, we don't need
to check widget focus but if text is empty.

Change-Id: I37f2fdb9e20a64ca38c61e60190f95635695c613
Reviewed-by: Aurélien Gâteau <agateau@kde.org>
Reviewed-by: David Faure (KDE) <faure@kde.org>