You CAN turn the auto-nulling ON, buts as JustSid said, it costs CPU.
Because then ANY new variable (both global and locals) will then requires CPU cycles to auto-null it.
To turn it on, insert this line VERY early in your script. By preferance, just after your #include "acknex.h" or "lite-c.h".
(Check under "#define" in the manual)
... hmmm ... looks like its only for local variables, and not globals as I thought ...
Ah well, give it a try anyway, it may just be a glitch in the manual regarding globals...