Null, here and in programming, refers to a lack of data. Typically, this means one of two things:
Current data is set to the value 0
A pointer is set to the value 0, ensuring it does not point toward any data.
It is worth noting, however, that in the context of pointers the N64 does not automatically invalidate attempts to jump to a NULL pointer. Instead, this is left to the programmers to determine what should be done. As is typical with most programmers, out of laziness no tests are done unless absolutely required. This means that a NULL pointer may, in fact, point toward 80000000 due to memory mirroring.