This is the pattern for the byte type:
Unless I'm missing something, this is equivalent to:
(I removed the ? and changed the repetition range from {1,2} to {0,2})
Meaning that it accepts 0x as a valid value.
Maybe that's intentional, but the weird combination of ? and {1,2} makes me think it might be a mistake. I also tried this with geth some weeks ago, and I think it didn't accept 0x as a valid value for the yParity of authorization items, although I haven't double-checked.