You can define this behavior if you want to. I did that on an embedded system where location zero was mapped and it was a complete pain in the fundament to arrange things so it would generate a fault. So I put on my wizard hat, intoned "Indirecting through NULL yields the value <some-reset-vector-or-something>, so mote it be!" and sure enough, a few weeks later one of the Q/A folks wrote a test that indirected through zero and filed a bug when it didn't explode.

"But it's supposed to generate an exception!"

"Says who?"

"Uh... it just does, right? It's in the CPU or something."

"Really? How do you think that works?" I took the opportunity to explain the mechanisms that were in play on various platforms, including ours, because the engineer in question had always treated indirection through zero as something just universally and magically fatal somehow, and there is no magic, just details.

It's still a darned good idea to keep the first 64K or 1MB or whatever of your address space unmapped (as well as similar guards at the top of your address space) because it catches interesting mistakes, but it's not like this stuff was handed down to us on stone tablets.