Fix AbstractClient::adjustedSize() for wayland windows with no committed buffer

AbstractClient::constrainClientSize() forces the minimum client size of
1x1.

If AbstractClient::adjustedSize() is called before the XdgToplevelClient
is mapped, it will return 1x1 rather than 0x0 as expected, which will
confuse

    QSize s = adjustedSize();
    if (s != size() && s.isValid())
        resizeWithChecks(s);

in AbstractClient::applyWindowRules(). Since 1x1 is different from 0x0,
the xdg-toplevel surface is going to be resized to 1x1.

BUG: 443705


(cherry picked from commit ff3465a8)