cutils: Allow NULL str in qemu_strtosz

All the other qemu_strto* and parse_uint allow a NULL str.  Having
qemu_strtosz not crash on qemu_strtosz(NULL, NULL, &value) is an easy
fix that adds some consistency between our string parsers.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230522190441.64278-13-eblake@redhat.com>