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

src/abstract_client.cpp

@@ -3600,7 +3600,12 @@ void AbstractClient::checkOffscreenPosition(QRect* geom, const QRect& screenArea
  */
 QSize AbstractClient::adjustedSize() const
 {
-    return clientSizeToFrameSize(constrainClientSize(clientSize()));
+    QSize size = clientSize();
+    // The client size is unknown until the window is mapped, don't constrain it.
+    if (!size.isEmpty()) {
+        size = constrainClientSize(size);
+    }
+    return clientSizeToFrameSize(size);
 }
 
 /**