ui: return NULL when getting cursor without a console

ui: return NULL when getting cursor without a console

VNC may try to get the current cursor even when there are no consoles
and crashes. Simple reproducer is qemu with -nodefaults.

Fixes: (again)
https://gitlab.com/qemu-project/qemu/-/issues/1548

Fixes: commit 385ac97f8 ("ui: keep current cursor with QemuConsole")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230428154807.2143652-1-marcandre.lureau@redhat.com>