wayland: don't call gdk_wayland_window_configure without valid size

Commit 7f12dc2b ("wayland: Postpone window resize requests until configured") runs gdk_wayland_window_configure unconditionally even if no valid size is set. This breaks GtkDialogs, which are not shown any more, and results in warnings like

gtk_widget_size_allocate(): attempt to allocate widget with width -3 and
height -3

Fixes: #2574 (closed)