response_get_{string,u64} include error handling for argument resp being
NULL but response_get_token does not handle this.

Make all three of response_get_{string,u64,token} handle NULL resp in
the same way.
Co-authored-by: NJonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Signed-off-by: NDavid Kozub <zub@linux.fjfi.cvut.cz>
Signed-off-by: NJonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Reviewed-by: NScott Bauer <sbauer@plzdonthack.me>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJon Derrick <jonathan.derrick@intel.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>