This works if you are only interested in the overall memory balance. However, if you want to make sure that all malloc() calls are matched by a free() call, you need to distinguish between NULL and a successfull zero-sized allocation, otherwise you run into troubles when you call free on an "actual" NULL pointer (which the standard defines as a no-op).
If malloc() had returned a real pointer, you'd have to free that too.
> wouldn’t it be better just to return NULL and guarantee that 0-sized allocations never use any memory at all?
Better: takes less memory Worse: blinds you to this portability issue.
A mismatch between malloc(0) and free(-1).
You’d know nothing about calls to malloc with non-zero sizes.