I don't see the answers directly addressing what i think you were asking, so here goes:
Yes, at least 1 address value is "wasted" (made unavailable for use) because of the constant used for null. Whether it maps to 0 in linear map of process memory is not relevant.
And the reason that address won't be used for data storage is that you need that special status of the null pointer, to be able to distinguish from any other real pointer. Just like in the case of ASCIIZ strings (C-string, NUL-terminated), where the NUL character is designated as end of character string and cannot be used inside strings. Can you still use it inside? Yeah but that will mislead library functions as of where string ends.
I can think of at least one implementation of LISP i was learning, in which NIL (Lisp's null) was not 0, nor was it an invalid address but a real object. The reason was very clever - the standard required that CAR(NIL)=NIL and CDR(NIL)=NIL (Note: CAR(l) returns pointer to the head/first element of a list, where CDR(l) returns ptr to the tail/rest of the list.). So instead of adding if-checks in CAR and CDR whether the pointer is NIL - which will slow every call - they just allocated a CONS (think list) and assigned its head and tail to point to itself. There! - this way CAR and CDR will work and that address in memory won't be reused (because it is taken by the object devised as NIL)
ps. i just remembered that many-many years ago i read about some bug of Lattice-C that was related to NULL - must have been in the dark MS-DOS segmentation times, where you worked with separate code segment and data segment - so i remember there was an issue that it was possible for the first function from a linked library to have address 0, thus pointer to it will be considered invalid since ==NULL