tee: client UUID: Skip REE kernel login method as well

Since the addition of session's client UUID generation via commit [1],
login via REE kernel method was disallowed. So fix that via passing
nill UUID in case of TEE_IOCTL_LOGIN_REE_KERNEL method as well.

Fixes: e33bcbab ("tee: add support for session's client UUID generation") [1]
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>