Why "or ah,ah"?

I'm slowly trying to get into osdev just to play around.

This tutorial has a piece of assembly that waits for a drive to become ready:

reset_drive:   mov ah, 0   int 13h   or ah, ah   jnz reset_drive

I get that after the interrupt, ah will be zero if the device is ready. But what does or ah,ah do? Seems a bit redundant... it appears to do nothing. (at least by my logic) What does it do?