Like many embedded cores, Cortex-M's don't have an MMU - everyone sees physical memory. In fact, I believe that's the chief distinguishing feature of the 3 core

Like many embedded cores, Cortex-M's don't have an MMU - everyone sees physical memory. In fact, I believe that's the chief distinguishing feature of the 3 core levels - Cortex-M's have no MMU, Cortex-A's have memory paging, and I think Cortex-R's are in between and have memory segments like MIPS. So if the board designer wired everything up to map RAM at address 0, then yes, you'd be able to dereference a pointer to address 0. Completely beside the point, though. Dereferencing NULL is always undefined behaviour in C.