NULL is never added to any array since NULL is a pointer value. Typically it is

#define NULL (void*(0))

THere is no way to get the size of an array unless it is statically allocated and the header that includes it defines the size or it is in the same module.

HEADER.H

extern int a[];

CCODE.C

int n = sizeof(a);  // Returns n = 4 (pointer size)

int b[10];

int m= sizeof(b); // Returns m= 10 * sizeof(int)