Having a stub or having a NULL pointer are two ways to leave something unimplemented. Which you use is an implementation detail. You can also use some other sigil instead of NULL.
A stub is an empty implementation. An empty implementation is different from no implementation, where there is no code to execute. It is the difference between writing 0 and writing nothing.
If you care about this distinction, yes. However the LWN post describes how both can be used to implement the same behaviour, that their is no defined special implementation.
I have no idea what LWN post you mean. You can make stubs trap by doing a NULL function pointer dereference, but that does not make NULL function pointer dereferences the same as stubs in general.
