I think it is, but for reasonable size problems, it probably doesn’t make a great deal of different. The operation count of gemv is notionally 2MN, the additional constants only add another 2M. You might expect 2MN >> 2M for anything other than trivially small cases, so the overall effect on computation time is probably not all that large.
Passing NULL probably won’t work, but passing the vector twice with beta = 0 should be safe.