NULL_WORD exists to provide an integral value representing null, primarily as a literal in x86 assembly code.  (There are a small number of uses outside of x86 assembly code, some of which might be considered questionable.)

The definition of NULL_WORD varies by platform.  It's also a macro.  It seems like a sufficient definition for all platforms would be

const intptr_t NULL_WORD = 0;

In particular, it really ought not ever be defined as equivalent to NULL.